test(integration): Check the enhance-image marker with python instead of bash

[oioki]

test_customizations checked for /created-by-enhance-image with bash -c 'if [ ! -e … ]; then exit 1; fi' in the web and sentry-cleanup containers. Use python -c "import os; assert os.path.exists(…)" instead, like the import ldap checks next to it. These were the last bash invocations inside sentry containers in the integration tests.

Prep for moving the sentry image to a distroless base, which has no bash.

[github-actions]

Coverage Results 📊

✅ 22 passed | Total: 22 | Pass Rate: 100% | Execution Time: 9m 32s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ Test counts unchanged from base.

All tests are passing successfully.

✅ Patch coverage is 100.00% (no changed executable lines found; target 50%).
Project statement coverage is 95.54% (unchanged from base (68a830f) to head (d71f844)).

Files with missing lines (1)

File	Coverage %	Lines
_integration-test/test_01_basics.py	99.51%	⚠️ 1 Missing

Coverage diff

@@            Coverage Diff             @@
##        master     #4541       +/-##
==========================================
  Coverage    95.54%    95.54%        —%
==========================================
  Files            5         5         —
  Tracked lines       336       336         —
  Branches         0         0         —
==========================================
  Hits           321       321         —
  Misses          15        15         —
  Partials         0         0         —

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