refactor(api): enable strict typing for rate limit test fixtures

[ShousenZHANG]

Summary

Part of #24645, following #44331.

tests/unit_tests/core/app/features/rate_limiting/conftest.py was excluded from strict Pyrefly checks and produced 34 diagnostics, all missing annotations. With this change it is under strict checking. test_rate_limit.py next to it stays excluded, because #43201 and #43044 are changing it.

While typing it, mock_time.increment() turned out not to work. It advanced a local counter, but the patched time.time kept returning the return_value it was created with (1000.0), so calling it never moved the clock. No test calls it; the tests set mock_time.return_value directly. It is removed.

mock_redis is unused on main, but #43201 uses it, so it stays and is typed. No production code changes.

What changed

• mock_redis. Its in-memory stores and fake Redis methods are typed. The TTL check uses isinstance(ttl, timedelta) instead of hasattr(ttl, "total_seconds"), which gives the same result for integer and timedelta TTLs.
• mock_time. It patches time.time with return_value=1000.0 and yields the mock, without increment.
• Return types. The other fixtures get them: Generator[...] for the yielding ones, and Callable[..., Generator[str]] for sample_generator.
• Config. Remove conftest.py from the unit-test Pyrefly exclusions.

Verification

• pyrefly check --config=tests/unit_tests/pyrefly.toml --use-ignore-files=false --disable-project-excludes-heuristics=true on conftest.py (from api/, Pyrefly 1.3.2 as pinned in uv.lock): 34 diagnostics before, zero after, with no suppressions.
• pyrefly check --config=tests/unit_tests/pyrefly.toml over the whole unit-test project: passed.
• These all passed:
  • ruff format --check and ruff check (without the cache), on the files and on the whole api/ tree
  • lint-imports: 68 contracts kept
  • scripts/check_no_new_getattr.py, check_no_new_controller_sqlalchemy.py, check_no_new_session_mock.py, check_no_new_base_model.py, check_no_new_test_doubles.py and check_no_spec_mock.py
  • dev/lint_response_contracts.py --fail-on-mismatch
• pytest -v tests/unit_tests/core/app/features/rate_limiting on Python 3.13: 28 passed, the same 28 test IDs as before the change. With this branch merged into feat: add workspace max active requests limit #43201's head, its rate limiting tests (31) pass.

Screenshots

Before	After
N/A: test and type-check configuration only; 34 strict diagnostics.	N/A: zero strict diagnostics; files included in normal type checks.

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've verified the change and added or updated tests where meaningful regression risk justifies coverage.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and vp staged (frontend) to appease the lint gods (backend only: ran the underlying ruff, import-linter, guard-script and Pyrefly commands listed above)

From Claude Code

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[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	75.77%	75.78%	+0.01%
Strict coverage	75.44%	75.45%	+0.01%
Typed symbols	63,293	63,298	+5
Untyped symbols	20,329	20,324	-5
Modules	3814	3814	0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.55%. Comparing base (38a5efe) to head (1688432).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #44391   +/-   ##
=======================================
  Coverage   88.54%   88.55%           
=======================================
  Files        5522     5522           
  Lines      331079   331163   +84     
  Branches    66207    66214    +7     
=======================================
+ Hits       293169   293254   +85     
- Misses      32724    32727    +3     
+ Partials     5186     5182    -4     

Flag	Coverage Δ
api	88.78% <ø> (+<0.01%)	⬆️

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