fix(preprocess): unify PR Newswire identities

[Sanjays2402]

Summary

PR Newswire observations now use the public build_news_identity helper, so callers can reproduce the identity stamped on collected documents. The focused regressions cover both discovery observations and collected documents.

Related Issue

Closes #110

Verification

• Regression tests failed against the unchanged source and passed with this fix.
• .venv/bin/python -m pytest -q --no-cov tests/preprocess/test_pr_newswire.py — 10 passed.
• Ruff format/check and lint-imports passed.
• basedpyright reports the same 12 unresolved optional-dependency imports with and without this diff in the minimal local environment; bash scripts/verify.sh therefore does not complete locally.
• The live PR Newswire smoke test was not run in the network-denied local test environment.

Checklist

• The title uses English Conventional Commit format: type(scope): summary.
• The related issue or design discussion is linked when applicable.
• bash scripts/verify.sh passes.
• Every applicable live-network component smoke test passes, or this PR states why none applies.
• Public behavior has focused tests, an example, and documentation where applicable.
• The PR is complete, small, and contains no unrelated changes.