Cover the MCP metadata tools and transport edges (closes #83, closes #85)

[kikashy]

Closes #83. Closes #85.

Both issues change internal/mcp/server_test.go, so they land together rather than conflicting in one file.

The metadata tools (#83)

Each success case asserts what makes the response useful, not that a response arrived:

• get_schema — sha256 and bytes must describe the schema text actually served. A digest that does not describe its payload is worse than no digest, because a caller pins against it. The served text is also asserted to be valid JSON.
• describe_runtime — identifies itself, lists supported versions, states artifact provenance — and its claimed versions must include the one get_schema really serves. Two payloads that disagree would otherwise both pass.
• test_conformance — the bundled corpus passes against its own runtime, with passed == total and the corpus digest and target version named.

Bad arguments are asserted to be tool errors inside successful JSON-RPC responses, never top-level protocol errors — the distinction that lets a client tell "your call was wrong" from "the transport broke".

The transport table (#85)

Pins what must and must not produce a response: parse error with a null id, blank and whitespace-only lines skipped, ping, the default protocol version, notifications and unknown notifications answered with nothing, and a valid request still processed after a malformed line.

Two mutations that initially did nothing

Worth recording, because the tests reported ok and that proved nothing:

Mutation	First attempt	Why
notification answers	no-op	reply() already returns false whenever a message has no id — breaking it requires bypassing reply entirely
get_schema digest wrong	no-op	had to corrupt the metadata after describe.Schema, not at the call site

Redone so they actually take effect, both fail correctly:

got 1 responses, want 0: [{"id":null,"jsonrpc":"2.0","result":{}}]
sha256 = "deadbeef", but the served bytes hash to "c65c559d..."

Ending the stream on a malformed line also fails, as it should.

Payload shapes read, not guessed

My first version asserted a schema field and an artifacts array that do not exist. I read the real payloads from a live server and rewrote against the actual contract — the schema text arrives as tool content, and provenance is a single artifactProvenance string.

go test ./internal/mcp and go test ./... pass; gofmt clean. Test file only.

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