feat: add Coder Agents chat tools to the MCP toolsdk (#28025)

Exposes the experimental Coder Agents chats API through the MCP tool
registry, so MCP clients (the hosted `/api/experimental/mcp/http` server
and `coder exp mcp server`) can start and drive server-side coding
agents.

New tools in `codersdk/toolsdk`, all thin wrappers over existing
`codersdk.ExperimentalClient` methods:

| Tool | Wraps |
|---|---|
| `coder_create_chat` | `CreateChat` (prompt, optional org, model
config, labels) |
| `coder_get_chat` | `GetChat` (status, last error, last turn summary,
workspace, files) |
| `coder_get_chat_messages` | `GetChatMessages` (user-facing parts,
chronological, cursor pagination, queued prompts) |
| `coder_send_chat_message` | `CreateChatMessage` (queue or interrupt
busy behavior) |
| `coder_interrupt_chat` | `InterruptChat` |
| `coder_archive_chat` | `UpdateChat` with `archived: true` |
| `coder_list_chat_model_configs` | `ListChatModelConfigs` (enabled
configs with default flag) |

Both MCP servers register tools from `toolsdk.All`, so no additional
wiring is needed. Responses are trimmed to what an MCP caller needs (IDs
as strings, user-facing transcripts) rather than full SDK payloads. No
new endpoints and no database changes.

Also adds MCP
[prompts](https://modelcontextprotocol.io/specification/2026-07-28/server/prompts)
for the chat workflows, defined once in `codersdk/toolsdk` and
registered by both servers:

| Prompt | Purpose |
|---|---|
| `coder_agents_delegate` | delegate a task to a Coder Agents chat and
monitor it to completion |
| `coder_agents_check` | check the status and recent activity of an
existing chat |

Each prompt declares the tools its workflow needs; the stdio server
skips prompts whose tools are excluded by `--allowed-tools`.

Tests run the tools against a chat-enabled coderdtest instance (fake
OpenAI-compatible provider plus in-process AI bridge), covering the full
lifecycle, an interrupt against a blocked turn, pagination cursors,
permission-dependent model config filtering, and argument validation.
Prompt coverage spans SDK rendering, the hosted
`prompts/list`/`prompts/get` round trip, and the stdio server including
allowlist gating.

> Mux created this PR on Mike's behalf.

coderd/mcp_http.go

@@ -80,6 +80,7 @@ func (api *API) mcpHTTPHandler() http.Handler {
 			if err := mcpServer.RegisterTools(authenticatedClient, toolOpt); err != nil {
 				api.Logger.Warn(r.Context(), "failed to register MCP tools", slog.Error(err))
 			}
+			mcpServer.RegisterPrompts()
 		case MCPToolsetChatGPT:
 			if err := mcpServer.RegisterChatGPTTools(authenticatedClient, toolOpt); err != nil {
 				api.Logger.Warn(r.Context(), "failed to register MCP tools", slog.Error(err))