Fix backend-only compile marker cleanup

[harsh21234i]

Closes #7089

Summary

Fix backend-only development runs leaving .web/nocompile, causing the
next full reflex run to skip frontend compilation.

Root Cause

run_backend() created the compile-skip marker unconditionally, including
when started with --backend-only.

Changes

• Only full-stack backend runs create .web/nocompile.
• Backend-only runs remove any stale marker.
• Added regression tests covering backend-only and full-stack behavior.
• Added a bugfix news fragment.

Testing

• git diff --check
• Ruff formatting check
• Added unit coverage in tests/units/utils/test_exec.py

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge because the marker lifecycle fix is directly covered for both backend-only and full-stack execution.

Summary

• Backend-only runs now remove stale compile-skip markers.
• Full-stack backend runs continue creating the marker.
• Regression tests independently verify marker creation and cleanup.

_{Reviews (2) · Last reviewed commit: "Cover backend marker creation in regress..."}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[codspeed-hq]

Merging this PR will not alter performance

✅ 40 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing harsh21234i:fix/7089-backend-only-nocompile (d810313) with main (9165dc6)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩