test: run freshly written stand-in executables once before relying on them

[sunerpy]

Summary

Main's Coverage job on 81bd7c7 (run 37143381765) failed once in
git_pending::tests::a_system_attributes_file_is_a_conversion_risk: left: None, right: Some(false). The cause is
an ETXTBSY race in the test helper, not in conversion_risk. The Coverage job is informational, and the required
CI Success passed.

The race:

1. The test writes a fake git shell script and runs it at once.
2. If another thread of the same test binary forks while the script is still open for writing, the child keeps the
   inherited descriptor until it execs.
3. Running the script meanwhile fails with ETXTBSY ("Text file busy").
4. git() maps a failed start to None, so conversion_risk returned None.

Two other tests share the pattern:

• git_pending_fast_path's hanging git. A failed start declines exactly the way the timeout does, so the test could
  pass without its timeout ever running.
• cli_ui's stand-in browser. If codegraph ui could not start it, the test would fail.

Each helper now starts the freshly written script once, retrying while the error is
io::ErrorKind::ExecutableFileBusy. Once a start succeeds no writer is left, so the test's own runs cannot hit it.

• The hanging git now runs exec sleep 30, so stopping it leaves no orphan.
• Its test now also checks that the 300 ms timeout elapsed.
• No product code changes.

Verification

• The mechanism, reproduced with a scratch program on this Linux host: 8 threads fork /bin/true while the main
  thread writes 3,000 fresh scripts and runs each at once. 313 of the 3,000 fail to start with ETXTBSY.
• The warm-up: with the start-once retry, 202 of 1,500 scripts needed a retry, and the 7,500 starts that
  followed had 0 ETXTBSY.
• The affected tests pass: codegraph-watch lib git_pending (1), --test git_pending_fast_path (27), and
  codegraph-rs --test cli_ui (15).
• The pre-push gate (make pre-ci) passed on this head: all Rust tests, 569 frontend tests, docs-check, clippy
  -D warnings with all features, and the archive smoke.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 63.15789% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/codegraph-watch/src/git_pending.rs	63.15%	7 Missing ⚠️

❌ Your patch check has failed because the patch coverage (63.15%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
- Coverage   95.07%   95.07%   -0.01%     
==========================================
  Files         198      198              
  Lines      109014   109033      +19     
==========================================
+ Hits       103647   103659      +12     
- Misses       5367     5374       +7     

Files with missing lines	Coverage Δ
crates/codegraph-watch/src/git_pending.rs	95.46% <63.15%> (-1.21%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.