fix(mcp): return -32601 for unknown methods, and implement the spec-required ping

[LifeInTheWeeds]

Independent of #12 — different file, no overlap, merges in either order.

Problem

The MCP stdio server has a single error path, so every failure — including "this method is not implemented" — is reported as JSON-RPC -32603:

_ => Err(anyhow!("Unknown method: {}", request.method)),
// ...later, unconditionally:
code: -32603, // Internal error

-32603 means the server itself failed. -32601 means the method is not implemented. Answering a capability probe with "I have failed internally" is a materially different signal, and a strict client may reasonably treat the server as unhealthy.

Three concrete effects, all reproduced against the stdio server on main:

Method	Before	Why it matters
ping	-32603	ping is REQUIRED by the MCP spec and is used for liveness checks. A client that pings during a long session gets an internal error, and the correct reaction to that is to drop the connection.
resources/list	-32603	Clients probe this after initialize even when the capability is not advertised.
prompts/list	-32603	Same.

After

ping            -> {"result":{}}
resources/list  -> {"result":{"resources":[]}}
prompts/list    -> {"result":{"prompts":[]}}
totally/bogus   -> {"error":{"code":-32601,"message":"Unknown method: totally/bogus"}}

Scope and honesty notes

This does not fix any connection failure. I found it while investigating one whose cause turned out to be entirely unrelated (a Windows DLL-path collision in my own environment). The spec violations are real on their own merits, but I want to be clear this is not a fix for a reported bug.

Build note: main does not currently compile against duckdb >= 1.10505.0, so this branch cannot be built or tested standalone until the first commit of #12 lands. The behaviour above was verified with that fix applied locally.

🤖 Generated with Claude Code

https://claude.ai/code/session_016pWGjWtzGf4UvtgiWqYQyR

[LifeInTheWeeds]

Both red checks here are expected, and neither is a defect in this change.

• Test / Clippy — this branch is cut from main, which doesn't compile against duckdb >= 1.10505.0. It cannot build standalone until the non_exhaustive wildcard fix (first commit of fix(duckdb): unbreak the build, make --engine duckdb reachable, and green up clippy #12) lands. I kept it branched from main so it stays single-purpose rather than bundling an unrelated fix.
• Clippy additionally hits the pre-existing unused_async drift that fix(clippy): allow unused_async on the synchronous engine impls #14 addresses.

Rustfmt was genuinely my error and is now fixed (force-pushed ca04708) — it passes.

Suggested order: #12 → #14 → #13. Happy to rebase this once the first two land.