fix(files): ignore empty segments in list extension filter

[harsh-thakkar7]

What

A comma-separated extension filter like md, (stray trailing comma) put an empty string into the extension allowlist, so extensionless files leaked into the listing even though only .md files were requested.

Observed:

• GET /api/files/list?extensions=md → ["a.md"]
• GET /api/files/list?extensions=md, → ["a.md", "noext"] (wrong)

Fix

Skip empty segments when building the allowlist, so md,, md, and ,md all behave exactly like md.

Tests

• tests/test_files_endpoints.py: new endpoint tests asserting the trailing-comma forms never return the extensionless file.
• Full backend suite passes (471 tests). Ruff clean.

[petertzy]

Approved — this is a necessary correctness fix. Empty comma-separated extension segments no longer cause extensionless files to leak into a filtered listing.
I also hardened normalization so segments such as . and .., which become empty after optional leading dots are removed, are discarded as well. Endpoint coverage includes trailing commas, whitespace, leading commas, and normalized-empty segments. Backend tests (487 passed), frontend tests (94/94), Ruff, ESLint, and TypeScript checks all pass.