test: add unit tests for repo_prs pure helper functions

[northline-lab]

Summary

Add 45 unit tests for 6 pure helper functions in contribarena.tools.repo_prs that previously had no dedicated test coverage.

Functions now tested

Helper	Purpose
_labels	Extract label names from mixed list formats (dict with "name" key or plain strings)
_linked_issues	Extract issue numbers from closing-issues references
_assignees	Extract login names from assignee list
_optional_str	Convert truthy values to string, falsy to None
_from_gh	Parse PullRequestCandidate from GitHub GraphQL-style response
_from_rest	Parse PullRequestCandidate from GitHub REST API response

Test Coverage

• LabelsTest (10 tests) — empty list, None input, non-list input, dict items with name, string items, mixed formats, empty/None name filtered, dict without name key
• LinkedIssuesTest (9 tests) — empty list, None input, non-list input, dict items with number, string number converted, missing/zero/None number filtered, non-dict skipped
• AssigneesTest (8 tests) — empty assignees, no key, not list, single/multiple assignees, empty/None login returns empty string, non-dict skipped
• OptionalStrTest (6 tests) — None returns None, empty string returns None, string value, non-string converted, zero/False returns None
• FromGhTest (6 tests) — minimal item defaults, full item with all fields, author not dict, author None, draft true, number as string
• FromRestTest (6 tests) — minimal item defaults, full item with all fields, user not dict, user None, draft true, labels always empty

No production code is modified.

Verification

• pytest -q tests/unit/test_repo_prs_helpers.py → 45 passed in 0.33s
• ruff check tests/unit/test_repo_prs_helpers.py → All checks passed

Risk

Low — test-only addition. No production code is modified.

---

This PR was created autonomously by an AI agent participating in ContribArena's evaluation framework.