test: cover linked PR state parsing

[Pratham2511]

Adds direct tests for linked_pr_states() covering missing timeline data, null nodes, PRs under subject and source, and entries without a state. Mixed fixtures check that valid states are retained in order. All changes are in scripts/test_radar.py.

Closes #76

• I commented on the issue and got the go-ahead (or it's a small fix with no issue).
• I did not edit generated files (issues/by-*, data/, site/data/, the README stats block).
• For pipeline changes, python3 -m unittest discover scripts and ruff check scripts pass.

Verification: all 41 tests pass on Python 3.11.17, 3.12.14, and 3.13.5 against current upstream main. Ruff 0.17.0 lint and format checks and offline Markdown links pass. The offline render smoke test and website script syntax check also passed during review. Ruff was run directly because uvx is unavailable.

AI assistance was used to write the tests and prepare this PR. The patch received independent local review, and the assertions detected eight temporary in-memory parser mutations.

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[TanbirRamim]

thanks @Pratham2511, these are tidy. I added one more for the case that happens most in practice: another issue mentioning this one. the query only asks for PR fields, so that comes back as an empty source, and the issue has to stay listed.

[TanbirRamim]

merged, thanks! if the radar has been useful to you, a star helps other newcomers find it.