fix(worker): fallback to latest known default-branch parent when provider ancestry isn't in DB

[nikosatwork]

Summary

This PR adds a default-branch fallback in worker parent selection when provider-reported ancestry cannot be matched to known commits in DB.

If the commit is on the repository default branch and normal ancestry lookup fails, worker now selects the latest earlier known commit on that same default branch.

Why

In some cases provider ancestry is unavailable or not yet represented in DB, which can leave parent selection unresolved even when there is clear recent default-branch history already known to Codecov.

This fallback reduces “missing parent/base” outcomes for default-branch commits while preserving existing behavior for non-default branches.

What changed

• repository.py
  • In fetch_appropriate_parent_for_commit, after existing parent/ancestor strategies fail, add a fallback:
    • only for default-branch commits
    • query same-repo, same-branch, non-deleted commits with non-null message
    • only commits earlier than current commit timestamp (when present)
    • select most recent by timestamp
• test_repository_service.py
  • Added test: default-branch commit falls back to latest previous known commit when ancestors are unknown
  • Added test: non-default branch does not use this fallback when ancestors are unknown

Scope

• Scoped to worker parent selection fallback and its repository service tests.
• Main risk is behavior change for default-branch parent selection when ancestry is unavailable.
• Mitigated by explicit positive/negative test coverage for fallback behavior.

Validation

• Targeted worker parent-selection tests pass.
• Lint and format checks were run per repository guidance.

Related

Closes codecov/feedback#861

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