test: improve coverage on critical paths (export, downloader, config)

[nikazzio]

Summary

The repo has no coverage gate in CI and no tracking of what critical paths lack tests. This issue tracks a targeted coverage push on the most important subsystems.

Areas to audit and cover

Area	Current state	Risk
services/export/service.py	Some tests exist but one hangs (#117); custom page selection, remote-temp flow, multi-item ZIP need more edge cases	High — user-facing export is a core flow
logic/downloader.py	Instantiation tested via test_downloader_pdf_priority; actual download flow, resume, cancel paths lightly covered	High — primary function of the app
logic/downloader_pdf.py	create_pdf() tested indirectly; OCR JSON loading (recently fixed to load_json), cover page generation, quality settings need direct tests	Medium
config_validation.py	4 tests; deprecated key warnings, whitelist prefix logic, nested schema validation could use more	Medium — bad validation = silent config drift
http_client.py	42 tests — good coverage; get_http_client() singleton + reset_http_client() need a test	Low — well tested
resolvers/discovery.py	Per-provider search tested via unit mocks; manifest validation, probe retry, edge-case payloads need more	Medium

Approach

1. Run pytest --cov=src --cov-report=term-missing to get a baseline
2. Identify uncovered critical branches (not just line coverage — focus on error paths and edge cases)
3. Add targeted tests for the gaps above
4. Consider adding a non-blocking coverage report to CI (ci: restore PR coverage for slow-marked networking and downloader tests #99)

Acceptance criteria

• Coverage baseline documented
• At least 80% line coverage on the 6 areas listed above
• No new test should be marked slow unless it truly requires network/filesystem I/O
• Coverage report added to CI as informational (non-blocking)

Related

• ci: restore PR coverage for slow-marked networking and downloader tests #99 (CI slow test coverage)
• fix: resolve 4 preexisting hanging tests in test suite #117 (hanging tests — must be fixed first for reliable coverage runs)