fix(logstore): normalize workflow datetimes to UTC

[weike-zhang]

Refs #40943

What

Normalize LogStore workflow timestamps to naive UTC in both workflow run and workflow node execution repositories.

Why

The current code uses local-time datetime constructors for epoch and missing-value branches, which can shift timestamps by host timezone and produce incorrect or even negative elapsed times.

How

• Use naive_utc_now() for missing timestamps
• Use ensure_naive_utc(datetime.fromisoformat(...)) for ISO strings
• Use datetime.fromtimestamp(..., tz=UTC).replace(tzinfo=None) for epoch values
• Add a regression test for mixed timestamp inputs

Verification

• uv run --project api pytest -o addopts='--import-mode=importlib' api/tests/unit_tests/extensions/logstore/repositories/test_logstore_api_workflow_run_repository.py api/tests/unit_tests/extensions/logstore/repositories/test_logstore_api_workflow_node_execution_repository.py api/tests/unit_tests/extensions/logstore/repositories/test_logstore_workflow_node_execution_repository.py

[github-actions]

Pyrefly Type Coverage

Metric	Base	PR	Delta
Type coverage	59.71%	59.71%	+0.00%
Strict coverage	59.29%	59.29%	+0.00%
Typed symbols	39,742	39,744	+2
Untyped symbols	27,010	27,010	0
Modules	3171	3172	+1

[weike-zhang]

The Web Full-Stack E2E failure is a CI infrastructure timeout: the Install Playwright browsers for core E2E step timed out after 15 minutes (https://github.com/langgenius/dify/actions/runs/32235372870/job/96014359616), before any test ran. Fork PRs can't re-run workflow runs without repo admin. Could a maintainer please re-run the failed job?

[weike-zhang]

This fix is already merged upstream via #40947 (identical changes to the workflow run and node-execution repositories). Closing this PR as superseded — no further action needed.