refactor: consolidate agent MCP onto a single persistent engine (#26599)

Two MCP code paths both spawned the servers declared in a workspace's
`.mcp.json`: the persistent engine in `agent/x/agentmcp` (which owns
tool-call execution via `CallTool`) and an ephemeral one-shot runner in
`agent/agentcontext` (`mcprunner.go`) that connected, listed tools, and
immediately closed each server purely for discovery. Every declared
server was launched twice, and the discovery path duplicated the
engine's `.mcp.json` parse, transport-build, env-resolve, and connect
logic.

This makes `agent/x/agentmcp` the single persistent MCP engine. The
`agentcontext` manager now reads that engine's per-server catalog
in-process through an injected `MCPCatalog` option and surfaces each
server as a `KindMCPServer` resource. The engine wires `SetOnReload` to
the manager's `Trigger`, so a reload (startup connect or `.mcp.json`
edit) re-resolves and re-pushes the pinned resources. Tool-call
execution is unchanged: it still flows through the engine's `CallTool`
over `POST /api/v0/mcp/call-tool`.

The now-dead HTTP discovery surface is removed: the agent `GET
/api/v0/mcp/tools` route with `agentmcp.API.handleListTools`, and
`workspacesdk.AgentConn.ListMCPTools` with `ListMCPToolsResponse` (mock
regenerated). The change nets roughly `-1370` lines, mostly the deleted
duplicate runner and its tests.

<details>
<summary>Decision log</summary>

The merge of #26585 made pinned `chat_context_resources` the sole source
of workspace context, which surfaced the duplicate spawning. Two options
were considered:

- **Option A + dependency injection (chosen):** keep `agent/x/agentmcp`
as the single persistent engine; `agentcontext` consumes its catalog
in-process and stays the orchestrator/owner at the API boundary (it
still pushes `KindMCPServer` resources). This is low-risk because
`agentcontext` already exposed the `resolver.MCPResources` seam, so the
change just rebinds it from the ephemeral runner to the shared engine.
- **Option B (rejected):** reimplement persistent pooling, reconnect,
singleflight, and race handling inside `agentcontext` and delete
`agentmcp`. Too broad, and it discards the engine's tested lifecycle for
no behavioral gain.

`agentcontext`'s discovery was never what kept servers alive; its runner
closed each server immediately after listing tools. The component
holding persistent connections was always `agentmcp`, which is why
execution already lived there. Consolidating onto it removes the
duplicated stack rather than a whole package: both packages survive with
distinct roles (`agentmcp` is the engine, `agentcontext` is the
orchestrator/owner).

</details>

Coder Agents generated on behalf of @kylecarbs

coderd/x/chatd/chatd_test.go

@@ -262,8 +262,6 @@ func newWorkspaceToolTestServer(
 	mockConn.EXPECT().SetExtraHeaders(gomock.Any()).AnyTimes()
 	mockConn.EXPECT().ContextConfig(gomock.Any()).
 		Return(workspacesdk.ContextConfigResponse{}, xerrors.New("not supported")).AnyTimes()
-	mockConn.EXPECT().ListMCPTools(gomock.Any()).
-		Return(workspacesdk.ListMCPToolsResponse{}, nil).AnyTimes()
 	mockConn.EXPECT().LS(gomock.Any(), gomock.Any(), gomock.Any()).
 		Return(workspacesdk.LSResponse{AbsolutePathString: "/home/coder"}, nil).AnyTimes()
 	mockConn.EXPECT().ReadFile(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).
@@ -898,17 +896,6 @@ func TestExploreChatUsesPersistedMCPSnapshot(t *testing.T) {
 	mockConn.EXPECT().ContextConfig(gomock.Any()).
 		Return(workspacesdk.ContextConfigResponse{}, xerrors.New("not supported")).AnyTimes()
 	workspaceToolName := "workspace-snapshot-mcp__echo"
-	mockConn.EXPECT().ListMCPTools(gomock.Any()).
-		Return(workspacesdk.ListMCPToolsResponse{Tools: []workspacesdk.MCPToolInfo{{
-			ServerName:  "workspace-snapshot-mcp",
-			Name:        workspaceToolName,
-			Description: "Workspace echo tool",
-			Schema: map[string]any{
-				"input": map[string]any{"type": "string"},
-			},
-			Required: []string{"input"},
-		}}}, nil).
-		AnyTimes()
 	mockConn.EXPECT().LS(gomock.Any(), gomock.Any(), gomock.Any()).
 		Return(workspacesdk.LSResponse{AbsolutePathString: "/home/coder"}, nil).AnyTimes()
 	mockConn.EXPECT().ReadFile(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).
@@ -3124,10 +3111,6 @@ func TestPersistToolResultWithBinaryData(t *testing.T) {
 		ContextConfig(gomock.Any()).
 		Return(workspacesdk.ContextConfigResponse{}, xerrors.New("not supported")).
 		AnyTimes()
-	mockConn.EXPECT().
-		ListMCPTools(gomock.Any()).
-		Return(workspacesdk.ListMCPToolsResponse{}, nil).
-		AnyTimes()
 	mockConn.EXPECT().
 		LS(gomock.Any(), gomock.Any(), gomock.Any()).
 		Return(workspacesdk.LSResponse{}, nil).
@@ -6478,14 +6461,11 @@ func toolMessageForTest(
 func setupToolExecutionAgentConn(
 	t *testing.T,
 	mockConn *agentconnmock.MockAgentConn,
-	mcpTools ...workspacesdk.MCPToolInfo,
 ) {
 	t.Helper()
 	mockConn.EXPECT().SetExtraHeaders(gomock.Any()).AnyTimes()
 	mockConn.EXPECT().ContextConfig(gomock.Any()).
 		Return(workspacesdk.ContextConfigResponse{}, xerrors.New("not supported")).AnyTimes()
-	mockConn.EXPECT().ListMCPTools(gomock.Any()).
-		Return(workspacesdk.ListMCPToolsResponse{Tools: mcpTools}, nil).AnyTimes()
 	mockConn.EXPECT().LS(gomock.Any(), gomock.Any(), gomock.Any()).
 		Return(workspacesdk.LSResponse{AbsolutePathString: "/home/coder"}, nil).AnyTimes()
 	mockConn.EXPECT().ReadFile(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).
@@ -9438,10 +9418,6 @@ func TestComputerUseSubagentToolsAndModel(t *testing.T) {
 	// for the initial screenshot check in the computer use path.
 	ctrl := gomock.NewController(t)
 	mockConn := agentconnmock.NewMockAgentConn(ctrl)
-	mockConn.EXPECT().
-		ListMCPTools(gomock.Any()).
-		Return(workspacesdk.ListMCPToolsResponse{}, nil).
-		AnyTimes()
 	mockConn.EXPECT().
 		ExecuteDesktopAction(gomock.Any(), gomock.Any()).
 		Return(workspacesdk.DesktopActionResponse{
@@ -10205,8 +10181,6 @@ func TestMCPServerToolInvocation(t *testing.T) {
 	mockConn.EXPECT().SetExtraHeaders(gomock.Any()).AnyTimes()
 	mockConn.EXPECT().ContextConfig(gomock.Any()).
 		Return(workspacesdk.ContextConfigResponse{}, xerrors.New("not supported")).AnyTimes()
-	mockConn.EXPECT().ListMCPTools(gomock.Any()).
-		Return(workspacesdk.ListMCPToolsResponse{}, nil).AnyTimes()
 	mockConn.EXPECT().LS(gomock.Any(), gomock.Any(), gomock.Any()).
 		Return(workspacesdk.LSResponse{}, nil).AnyTimes()
 	mockConn.EXPECT().ReadFile(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).
@@ -10714,8 +10688,6 @@ func TestMCPServerOAuth2TokenRefresh(t *testing.T) {
 	mockConn.EXPECT().SetExtraHeaders(gomock.Any()).AnyTimes()
 	mockConn.EXPECT().ContextConfig(gomock.Any()).
 		Return(workspacesdk.ContextConfigResponse{}, xerrors.New("not supported")).AnyTimes()
-	mockConn.EXPECT().ListMCPTools(gomock.Any()).
-		Return(workspacesdk.ListMCPToolsResponse{}, nil).AnyTimes()
 	mockConn.EXPECT().LS(gomock.Any(), gomock.Any(), gomock.Any()).
 		Return(workspacesdk.LSResponse{}, nil).AnyTimes()
 	mockConn.EXPECT().ReadFile(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).
@@ -12670,8 +12642,6 @@ func setupWorkspaceContextAgentConn(
 			}, nil
 		},
 	).AnyTimes()
-	mockConn.EXPECT().ListMCPTools(gomock.Any()).
-		Return(workspacesdk.ListMCPToolsResponse{}, nil).AnyTimes()
 	mockConn.EXPECT().LS(gomock.Any(), gomock.Any(), gomock.Any()).
 		Return(workspacesdk.LSResponse{AbsolutePathString: "/home/coder"}, nil).AnyTimes()
 	mockConn.EXPECT().ReadFile(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).