(mcp) Add language-neutral MCP 2026 conformance vectors

[tcballard]

Priority

P2 — language-neutral MCP compatibility assurance.

Problem

The current MCP 2026 test suite is predominantly a small set of implementation-local happy/error examples. Core's full decided-mcp suite can pass while invalid Origin, version-carrier conflict, invalid JSON-RPC version/ID, and incomplete audit behaviors remain accepted.

AsDecided needs small, stable, language-neutral contract fixtures in asdecided/spec so current and future server implementations share the same positive and negative compatibility oracle.

Scope

Create a pinned MCP compatibility fixture set covering:

• legacy 2025-11-25 frozen behavior;
• current 2026-07-28 discovery, metadata, capabilities, cache hints, and result typing;
• JSON-RPC envelope validity;
• HTTP/body version agreement;
• required current headers and header/body agreement;
• Origin policy outcomes;
• notifications and request IDs;
• structured error codes and HTTP statuses;
• tool-schema locality and object-root constraints.

Audit JSONL semantics should remain a Core integration concern, but the protocol-level expected wire outcomes belong here.

Acceptance criteria

• Fixtures are small, deterministic, language-neutral, and versioned.
• Each vector identifies transport, input request/headers, and exact or schema-constrained expected output.
• Both valid and invalid cases are represented.
• Core consumes the fixtures in CI rather than copying them.
• Legacy byte fixtures remain explicitly separate from current semantic/schema vectors.
• The suite documents its relationship to the official MCP 2026-07-28 specification.
• Adding a future implementation requires no Python oracle.