test: add regression tests for bugs fixed in #537–#546

[ayushshukla1807]

Adds test_regression_recent_fixes.py with one test class per recently-fixed bug.

Each class is named after the issue it guards, making it trivial to trace a future failure back to the original report:

Class	Guards Against
TestDownloadEntriesCsvEmptyRound	#531 -- IndexError on empty CSV export
TestFaveCrossCampaignCollision	#526/#527 -- fave cross-campaign state collision
TestNoPdbInProductionCode	#523 -- active pdb breakpoints in production modules
TestSlugifyNullSafety	#542 -- slugify crash when rnd.name is None
TestLoadPartialCsvFallback	#517 -- load_partial_csv passed wrong type

The source-inspection tests (pdb, bare except, null-guard) are intentionally documentation-style: if someone re-introduces the bug in the future, the test fails with an explicit message pointing back to the issue number.