Add coverage for comparability._repo_key's sorted-keys fallback

[galuis116]

benchmark/comparability._repo_key picks a per-repo row's identity from repo_path/url/repo/name, then falls back to freeze_commit[:10], and finally to repr(sorted(entry.keys())) for a row with none of those. That final fallback (line 123) is never exercised by the test suite, so a row with no identity field — which must still get a stable, deterministic key rather than collide or raise — is unverified.

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Add a _repo_key test asserting the identity-field precedence, the freeze_commit fallback, and the sorted-keys fallback for an identity-less row. Tests only.

[vanguarstew]

This fits the roadmap as a low-risk benchmark hardening task — it exercises an untested fallback in the comparability layer that could otherwise produce unstable keys or collisions. Tests only, no agent/ or Polaris surface touched, so it's in scope and doesn't conflict with the #1935 track. Please add the test asserting identity-field precedence, freeze_commit fallback, and sorted-keys fallback for an identity-less row, and ensure it fails without the current implementation. Next step: open a PR referencing this issue; it will pass the automated gates and be reviewed on the standard rubric.