CI smoke tests for external tool compatibility regression detection

[MPCoreDeveloper]

Summary

Add CI smoke tests for core external-tool compatibility scenarios.

Why

Compatibility can regress silently without automated checks.

Scope

• Add automated checks for protocol handshake, simple query, and metadata discovery patterns.
• Cover psql/libpq baseline and scripted client interactions.
• Track compatibility regression signals in CI output.

Implementation Plan

1. Define minimal, deterministic compatibility test suite.
2. Implement harness scripts and CI workflow integration.
3. Add pass/fail gates and artifact publishing.
4. Document how to run locally.

Acceptance Criteria

• CI runs compatibility smoke checks on target branches.
• Regressions surface with actionable logs.
• Local reproduction workflow documented.

Dependencies

• Depends on protocol and metadata baseline.