fix(scoring): gate valid merged PRs on source token score

[Dexterity104]

Summary

The OSS scoring path measured one threshold constant against two different quantities. The base-score gate in calculate_base_score_for_pr_files keyed off the SOURCE-only token score, while the eligibility "valid merged PR" gate in check_eligibility keyed off pr.token_score, the all-category aggregate (SOURCE + TEST). As a result a merged PR could earn zero source base score yet still count toward min_valid_merged_prs, the gate that unlocks all scoring for a repository. A near-threshold source PR plus a small test file was enough to flip a PR from "not valid" to "valid".

This change makes validity match the base-score gate:

• BaseScoreResult now returns source_token_score (already computed inside the helper, so no extra work).
• ScoredPR stores source_token_score alongside the aggregate token_score, set during score_pr.
• check_eligibility counts a merged PR as valid only when its source_token_score clears the threshold.
• The constant comment and the check_eligibility docstring now describe the actual behavior.

The aggregate token_score is still kept for reporting rollups, the storage adapter, and the issue discovery cache, so both fields remain in use. The issue discovery path is intentionally untouched: it gates on a separate constant (min_token_score_for_valid_issue) compared consistently against the aggregate, so it has no internal mismatch.

Related Issues

Fixes #1396

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• Tests added/updated
• Manually tested

Added test_validity_gate_keys_off_source_not_aggregate_token_score, which confirms a repo of merged PRs that pass the aggregate threshold but fail the SOURCE threshold is ineligible. Updated the _merged test helper to set the gating field, extended the base-score helper test to assert source_token_score is surfaced, and updated the BaseScoreResult stubs in test_scoring.py. The eligibility gate logic was verified directly: SOURCE-fail rows produce an ineligible result and SOURCE-pass rows produce an eligible one.

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[anderdc]

The eligibility gate this patches no longer exists on test. check_eligibility now counts merged PRs directly against min_valid_merged_prs and no longer compares token_score against min_token_score_for_base_score, so the SOURCE-vs-aggregate mismatch from #1396 is already gone and this diff conflicts against the current path. Closing as out of date.