Compatibility test harness

[g2vinay]

Replace the old handshake-based tests with RC-native integration coverage.

Work items:

1. Update hardcoded protocol-version strings in the test suite.
2. Replace initialize/initialized-based smoke tests with RC-style request tests.
3. Add HTTP-mode tests that validate the new header contract.
4. Add elicitation round-trip tests that exercise the new multi-round flow.
5. Add protocol compatibility tests for single, namespace, all, and consolidated modes.
6. Add interop tests for external MCP server discovery and proxying.
7. Add regression tests for the removal or replacement of session-affinity assumptions.

Acceptance criteria:

• The test suite covers the same user-visible behaviors without depending on removed protocol primitives.
• CI fails if the server regresses to the legacy handshake model.