MCP tool handlers do not accept or propagate CancellationToken from the client

[Widthdom]

Summary

None of the MCP tool handlers accept or propagate a CancellationToken. JSON-RPC $/cancelRequest notifications from the client are effectively ignored — long-running impact_analysis, index, or large search calls run to completion even after the client cancels (or disconnects).

Evidence

• src/CodeIndex/Mcp/McpServer.cs dispatches each tool call synchronously and does not maintain a per-request CancellationTokenSource.
• src/CodeIndex/Mcp/McpToolHandlers.cs — every Execute* method is synchronous (JsonNode Execute*(...)) with no CancellationToken parameter.
• Verified codebase-wide: a search for catch (OperationCanceledException) returns zero hits, confirming there is no cooperative cancellation anywhere downstream of MCP handlers.

Impact

• A user pressing Ctrl-C in their IDE-side MCP client cannot interrupt a multi-minute indexing or analysis call.
• Disconnected clients leave the server doing wasted work.
• This is a prerequisite for the cancellation issues in IndexCommandRunner.RunFullScan and DbWriter backfills (related issues filed separately).

Proposed direction

1. Maintain a ConcurrentDictionary<requestId, CancellationTokenSource> in McpServer.
2. On $/cancelRequest, look up the request id and cancel the token.
3. Plumb CancellationToken through all Execute* methods and into DB / file walks.
4. Wrap each handler invocation in a try { ... } catch (OperationCanceledException) { return CreateCancelledResponse(id); }.

Repro env

• Branch: main @ 2ee912d (release v1.21.0)

[Widthdom]

Work started on branch fix-issue1418.