fix(discovery): TransactionTestCase isolation via toxic queue routing

[NikkeTryHard]

Summary

• Mark @pytest.mark.django_db(transaction=True) tests as toxic so workers exit after running them
• Process-death provides DB isolation — no flush/truncate needed in fork-based architecture
• Close stale connections for transaction tests before returning from isolation setup

Closes #82

How it works

Tests with transaction=True commit directly to the database, bypassing Django's savepoint isolation. Instead of implementing Django's flush command (expensive, complex), we leverage tach's existing toxic queue:

1. Scanner already parses transaction=True into MarkerInfo
2. New: has_django_transaction_marker() on RunnableTest checks for this marker
3. New: Toxicity tagging in main.rs ORs the existing graph-based check with the marker check
4. Result: Transaction tests → toxic queue → worker exits after test → clean DB from Zygote snapshot

Files Changed

File	Change
src/discovery/resolver.rs	Add has_django_transaction_marker() + 3 unit tests
src/main.rs	OR marker check into both toxicity tagging locations
src/tach_harness.py	Close stale connections for transaction=True path

Verification

• cargo build ✅ | cargo fmt ✅ | cargo clippy -D warnings ✅ | 857/857 tests pass ✅
• Docker verified

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/issue-82-transaction-test-isolation

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