fix(rmcp): enforce strict stateless initialization errors

[jmagar]

Summary

Follow-up to #3, found while reviewing Labby PR dinglebear-ai/labby#650. Limit the fix to the opt-in strict stateless HTTP path; relaxed stateless and stateful legacy initialization remain compatible.

• Require the protocol-version header on strict initialize requests.
• Validate modern Mcp-Method on strict initialization without changing legacy exemptions elsewhere.
• Map strict terminal protocol errors to the existing modern HTTP status codes.
• Preserve the parsed JSON-RPC request ID and HeaderMismatch envelope for malformed protocol headers.

Only the HTTP transport implementation and its focused protocol-version tests change. Labby consumes the immutable branch commit; this PR does not modify modelcontextprotocol/rust-sdk upstream.

Validation

• 61 focused SDK tests passed: protocol versions, standard headers, negotiation, stateless compatibility, and JSON/SSE responses.
• Malformed-header regression failed before the preflight fix and passed afterward.
• Independent code, compatibility, error-envelope, and test review is clean after fixes.
• Labby integration validation is recorded in fix(mcp): enforce transport lifecycle contracts and document oracle workflows labby#650.

The all-features configuration enables local and skips these non-local tests. The executed feature set was client,transport-streamable-http-server,transport-streamable-http-client-reqwest,reqwest; no zero-test run is counted as evidence.