[MB-0505][P1] Add Matrix integration tests for routing and replies

[FarisZR]

Summary

Build integration coverage for matrix room/direct routing and reply bridge behavior.

Scope

• Cover room/direct routing, command handling, and reply-to-comment bridge.

Implementation Notes

• Follow long-term modernization rules in docs/plans/2026-02-21-refactor-long-term-modernization-backlog-plan.md.
• Do not add short-term hardening if replacement is the target architecture.
• Keep changes PR-sized and reversible.

Acceptance Criteria

• Matrix routing and reply paths are regression-protected.

Test Requirements (Mandatory)

• Integration tests against mocked matrix client APIs or local test homeserver harness.

Dependencies

• MB-0501,MB-0302,MB-0303

Definition of Done

• All acceptance criteria met.
• Required tests added and passing in CI.
• Documentation/config updates included where relevant.
• Backward-compatibility or migration notes added if behavior changes.

References

• Plan: docs/plans/2026-02-21-refactor-long-term-modernization-backlog-plan.md
• Backlog ID: MB-0505