fix: apply MCP server selection when editing a chat message - #28471

Problem

Editing a user message in an Agent chat silently dropped the MCP server selection. The composer renders the MCP picker in edit mode and toggles update local client state, so the picker displayed the new selection, but the edit request omitted mcp_server_ids at every layer (frontend request builder, codersdk.EditChatMessageRequest, the PATCH /chats/{chat}/messages/{message} handler, and chatd.EditMessage). The chat's persisted selection never changed and the regenerated turn ran without the newly enabled MCP tools. Two dogfood users hit this within hours; there is no error anywhere and the UI shows the opposite of the server state.

Changes

• codersdk: add MCPServerIDs *[]uuid.UUID to EditChatMessageRequest, mirroring CreateChatMessageRequest (nil preserves the current selection).
• chatd: extract the send path's MCP update block into one shared applyRequestedMCPServerIDs helper (explore-subagent snapshot immutability guard plus Force On enforcement, Cure53 CDM-02-010) and call it from both SendMessage and EditMessage, so enforcement cannot drift between the two paths.
• coderd: extract the send handler's request validation (dedupe, enabled-in-organization check, persisted-ID exemption) into normalizeRequestedChatMCPServerIDs and wire it into the edit handler, which now threads the selection into EditMessageOptions.
• Frontend: the edit request now includes mcp_server_ids: [...effectiveMCPServerIds], exactly like the send path, making the picker's displayed state real.
• make gen artifacts (swagger, API docs, typesGenerated.ts).

Tests

Each layer is covered and was proven with independent red toggles (removing one layer's wiring fails only that layer's tests):

• chatd (TestEditMessage_MCPServerIDs): edit applies a provided selection, nil preserves it, an emptied list cannot remove a force_on server, and explore subagent chats keep the spawn-time snapshot.
• API (TestPatchChatMessage/MCPServerIDsApplied, MCPServerIDsInvalidRejected): persistence via the endpoint, omission preserves, unknown IDs get the same 400 as the send path.
• Storybook (EditAppliesMCPServerSelection): toggling a server on during an edit puts it in the edit request payload.

Note: the AgentChatPage.stories.tsx story "Queued For Capacity After Polling" fails locally on current main as well (verified against the main baseline with this branch's changes reverted); it is unrelated to this diff.

Remote dogfood UAT ran on the exact head and passed, including proof that the regenerated turn actually gains the newly enabled MCP server's tools.

🤖 Xum acted on Mike's (@ibetitsmike) behalf. • Model: anthropic:claude-fable-5