test: add unit tests for repo_eligibility pure helpers

[northline-lab]

Summary

Add 36 unit tests for four pure helper functions in contribarena.tools.repo_eligibility that previously had no dedicated test coverage.

Test Coverage

ParseGithubDatetimeTest (5 tests)

• ISO format parsing with Z suffix and timezone offsets
• Invalid format handling
• UTC timezone conversion
• Edge cases (empty strings, various formats)

LooksEnglishTest (8 tests)

• ASCII ratio calculation and threshold behavior
• 85% boundary conditions (exactly at, above, below)
• Long text truncation at 2000 characters
• Whitespace and empty string handling

ProhibitsAiOrBotsTest (10 tests)

• All six prohibited phrase variants detected
• Case-insensitive matching
• Partial phrase rejection (e.g., "AI tools" doesn't trigger)
• Empty text handling

IsExternalRecentPrTest (13 tests)

• External contributor detection
• Owner exclusion (case-insensitive)
• Bot account filtering ([bot] suffix)
• Time boundary handling (exactly at, after)
• REST API format support (merged_at vs mergedAt)
• Missing/invalid data handling

Why This Matters

These pure helper functions are critical for:

• Determining repository eligibility during discovery
• Filtering bot/owner PRs from external contribution counts
• Detecting repositories that prohibit AI contributions
• Parsing GitHub's various datetime formats

Having comprehensive tests ensures the eligibility checker behaves correctly across edge cases.

Verification

pytest tests/unit/test_repo_eligibility.py -v
# 36 passed

ruff check tests/unit/test_repo_eligibility.py
# All checks passed!

Changes

• No production code changes - test file only
• Added tests/unit/test_repo_eligibility.py with 4 test classes