chore(tests): replace synthetic s1/sub1 subscription_id literals with real UUIDs in test fixtures

Start in tests/news/test_flask_api_deep_coverage.py and review the listed subscription test fixtures, URLs, and mock assertions. Run the focused pytest file and pre-commit for that file after the mechanical fixture migration. Done means the valid UUID constant is used consistently, exact assertions remain intact, and the specified searches find no remaining synthetic subscription IDs.

Summary

Several news-route coverage tests use s1 and sub1 as fake subscription IDs. PR #5603 already forced one fix on get_news_feed; #5627 will force another on get_subscription_history. Future guards on any other subscription_id-bearing route will collide with the same fixtures. Migrate the synthetic literals to real UUIDs proactively so the next guard lands without test-fixture churn.

Evidence

grep -nE '/s1|subscription_id.*=|"s1"' tests/news/test_flask_api_deep_coverage.py on the pre-#5603 tree surfaces the following occurrences:

Why this is worth doing now

• #5627 will require migrating the three history-row calls; doing them in isolation leaves s1 brittle for every other route's future guard.
• Every s1 literal in a URL path or mock assertion will be the first thing any new validator trips; the same refresh would recur.
• One mechanical pass keeps the diff small and easy to review.

Proposed change

Mechanical fixture migration only. No production-code change.

• Replace every synthetic s1 and sub1 in tests/news/test_flask_api_deep_coverage.py with a single fixed valid UUID literal, e.g. 11111111-1111-4111-8111-111111111111 (mirroring PR #5603).

• Update mock fixtures ({"id": "s1", …}) and assertions (mock_X.assert_called_once_with("s1", …)) to use the UUID consistently.

• Inside tests/news/test_flask_api_deep_coverage.py, introduce a single module-level constant:

  _TEST_SUB_ID = "11111111-1111-4111-8111-111111111111"
  

  and reference it everywhere s1 would have gone. Keeps the migrations trivially greppable and future migrations a one-line change.

Acceptance criteria

• grep -nE '/s1|subscription_id.*=|"s1"' tests/news/test_flask_api_deep_coverage.py returns no matches except those already migrated by PR #5603.
• No test assertion weakened (e.g., do not switch from exact equality to substring / in).
• pytest tests/news/test_flask_api_deep_coverage.py -q is green in the repo venv (Python 3.12) and via the project's Docker test image.
• pre-commit run --files tests/news/test_flask_api_deep_coverage.py is green.
• Search for s1/sub1 across tests/ shows no other surprising subscription_id occurrences.

Out of scope

• Any production-code guard addition (covered by #5627 and future issues).
• Changing test class names, fixtures, or assertion structure beyond the literal swap.
• Any other test file that uses s1 for unrelated reasons (e.g., a search query, a subscription topic) — those remain as-is.

References

• PR #5603 — original sub1 → UUID migration in TestGetNewsFeed.test_with_query_params (commit 517331e8c). Same diff shape is being generalized here.
• Issue #5627 — adds the same fixture migration for the history route; this issue fills in every other call site so #5627 only needs to cover the history rows.
• File: tests/news/test_flask_api_deep_coverage.py.

Estimated complexity

Small. Mechanical edit, ~25 minute budget. No schema or contract change.