fix(mars): route verification and refinement using matching agent IDs

[liuzhengyang699]

MARSAgent uses integer IDs, but generate_solution stores them as strings. The verifier compares these values directly, so agent 0 can verify its own answer, the temperature-diversity preference cannot find the author, and both improvement paths silently skip generated solutions because no original agent matches.

Compare IDs as strings when selecting a verifier and locating the author for refinement. This preserves integer-ID compatibility and the existing fallback for aggregated solutions or a single agent.

The regressions use real generation, verification, and improvement methods with mocked model responses. They cover all three authors in sequential and parallel mode, checking the selected peer, feedback passed to the author, updated answer, and token accounting. All six pipeline cases fail before the fix.

Validation: python -m pytest tests/test_mars_agent_routing.py tests/test_mars_parallel.py tests/test_mars_workspace_verification.py tests/test_mars_aggregation_sampling.py -q passed 26 tests and 4 subtests. Ruff on the new tests and git diff --check passed. No live model calls.