Add MCP protocol compatibility fixtures

[ocularminds]

Context

The MCP smoke test verifies initialization, tool inventory, and an evaluate round trip. Compatibility drift across supported MCP clients and protocol revisions is not represented as versioned fixtures.

Scope

Create protocol fixtures that exercise the supported handshake and tool-call envelopes, including invalid and forward-compatible inputs.

Acceptance criteria

• Fixtures identify the MCP protocol revision and client behavior they represent.
• Supported initialization and evaluate requests produce stable, schema-valid responses.
• Unknown fields are handled according to the protocol contract.
• Malformed requests fail closed with bounded, non-sensitive errors.
• The fixture suite runs against the built container in CI.