allow initialization after unsupported mcp discovery

[tylergibbs1]

fixes #1449

executor mcp forwarded server/discover to the local streamable http endpoint before a session existed. the endpoint returned -32000 Server not initialized, and the stdio bridge closed before clients could fall back to initialize.

the bridge now answers that one unsupported pre-initialize method with -32601 Method not found. all other messages still pass through to the daemon, and requests after initialization are unchanged.

the raw stdio integration test sends discovery, initializes on the same connection, lists execute, and calls it successfully.

validation

• bun run format:check
• bun run lint
• bun run typecheck
• bun run test