fix: inherit MCP server IDs from parent chat when spawning subagents (#23571)

Child chats created via `spawn_agent` and `spawn_computer_use_agent`
were not inheriting the parent's `MCPServerIDs`, meaning subagents lost
access to the parent's MCP server tools.

## Changes

- Pass `parent.MCPServerIDs` in the `CreateOptions` for both
`createChildSubagentChat()` and the `spawn_computer_use_agent` tool
handler in `coderd/x/chatd/subagent.go`.

## Tests

Added 3 tests in `subagent_internal_test.go`:
- `TestCreateChildSubagentChat_InheritsMCPServerIDs` — verifies child
chat gets parent's MCP server IDs (multiple servers)
- `TestSpawnComputerUseAgent_InheritsMCPServerIDs` — verifies computer
use subagent gets parent's MCP server IDs via the tool
- `TestCreateChildSubagentChat_NoMCPServersStaysEmpty` — verifies no
regression when parent has no MCP servers

coderd/x/chatd/subagent.go

@@ -326,6 +326,7 @@ func (p *Server) subagentTools(ctx context.Context, currentChat func() database.
 					ChatMode:           database.NullChatMode{ChatMode: database.ChatModeComputerUse, Valid: true},
 					SystemPrompt:       computerUseSubagentSystemPrompt + "\n\n" + prompt,
 					InitialUserContent: []codersdk.ChatMessagePart{codersdk.ChatMessageText(prompt)},
+					MCPServerIDs:       parent.MCPServerIDs,
 				})
 				if err != nil {
 					return fantasy.NewTextErrorResponse(err.Error()), nil
@@ -393,6 +394,7 @@ func (p *Server) createChildSubagentChat(
 		ModelConfigID:      parent.LastModelConfigID,
 		Title:              title,
 		InitialUserContent: []codersdk.ChatMessagePart{codersdk.ChatMessageText(prompt)},
+		MCPServerIDs:       parent.MCPServerIDs,
 	})
 	if err != nil {
 		return database.Chat{}, xerrors.Errorf("create child chat: %w", err)