[ci] Address copilot review feedback on import-time check

- determine-jobs: trigger on changes to requirements_test.txt too; that
  file is hashed into the venv cache key and installed during
  restore-python, so a change there can alter the import environment.
- ci.yml: merge the --check and --har steps so we only run
  importtime-waterfall once per job (was measuring twice: ~7-8s of
  wasted CI time per run). Uses the new script/check_import_time.py
  --check --har <path> combination; the HAR reflects the same
  measurement that produced the pass/fail decision.
- script/check_import_time.py: refactor the CLI so --har is a standalone
  option rather than a mutually-exclusive mode. --check and --update
  each accept an optional --har PATH that writes the HAR from the same
  subprocess invocation. Plain --har is still supported for local use.
- tests: add tests/script/test_check_import_time.py covering HAR parsing,
  root lookup, offender ranking/dedup, budget round-trip, and the three
  --check exit paths (pass, regression, missing budget) plus the new
  --check --har combined write. Add requirements_test.txt case to the
  should_run_import_time parametrized test.

.github/workflows/ci.yml

@@ -123,21 +123,17 @@ jobs:
         with:
           python-version: ${{ env.DEFAULT_PYTHON }}
           cache-key: ${{ needs.common.outputs.cache-key }}
-      - name: Check import time against budget
+      - name: Check import time against budget and write waterfall HAR
         run: |
           . venv/bin/activate
-          script/check_import_time.py --check
-      - name: Generate waterfall HAR
-        if: always()
-        run: |
-          . venv/bin/activate
-          script/check_import_time.py --har importtime.har
+          script/check_import_time.py --check --har importtime.har
       - name: Upload waterfall HAR
         if: always()
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
           name: import-time-waterfall
           path: importtime.har
+          if-no-files-found: ignore
           retention-days: 14
 
   pytest: