fix: stabilize WelcomeServiceStatusProbe RunCommandAsync test

[JerrettDavis]

Summary

• Root cause: Process.WaitForExitAsync does not guarantee that redirected stdout/stderr pipe data has been fully delivered to the async reader tasks before it returns. This is a documented .NET behaviour — unlike the synchronous WaitForExit() overload, the async version only waits for the process to exit, not for I/O completion on the redirected streams.
• The race: On Linux CI runners (which are more resource-constrained and schedule OS pipe flushes later), the sequence was: process exits → WaitForExitAsync unblocks → code awaits outputTask/errorTask → reads empty/partial strings before the OS has delivered all buffered pipe data. The test then saw Output not containing "hello" or Error not containing "oops".
• The fix: Add process.WaitForExit() (no-arg synchronous overload) immediately after WaitForExitAsync returns. Microsoft's documentation explicitly states: "To ensure that asynchronous event handling has been completed, call the WaitForExit() overload that takes no parameter after receiving a true return value from this overload." This drains all pending pipe data before the reader tasks are consumed.

Test plan

• RunCommandAsync_WhenCommandSucceeds_CapturesExitCodeOutputAndError — ran 10 consecutive times locally, 10/10 passed
• Full WelcomeServiceStatusProbeExtendedTests class — all 31 tests pass, 0 regressions
• CI green on this PR
• Dependabot PRs chore(deps): bump softprops/action-gh-release from 2 to 3 #484, Bump Azure.Security.KeyVault.Secrets from 4.9.0 to 4.10.0 #485, Bump coverlet.collector from 8.0.1 to 10.0.0 #486 will pick up the fix automatically when they rebase onto main

Note: The flake does not reproduce locally on Windows (Windows pipe flushing semantics differ), but the hazard is real and the fix is correct per the .NET Process documentation. The fix is purely additive — WaitForExit() after WaitForExitAsync is a no-op when the process has already exited and streams are already drained.

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Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

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Code Coverage

Summary
  Generated on: 04/18/2026 - 15:44:43
  Coverage date: 04/18/2026 - 15:42:50 - 04/18/2026 - 15:44:34
  Parser: MultiReport (7x Cobertura)
  Assemblies: 23
  Classes: 1067
  Files: 591
  Line coverage: 74.6%
  Covered lines: 36826
  Uncovered lines: 12486
  Coverable lines: 49312
  Total lines: 97268
  Branch coverage: 63.6% (14339 of 22512)
  Covered branches: 14339
  Total branches: 22512
  Method coverage: 86.4% (6317 of 7308)
  Full method coverage: 74.2% (5426 of 7308)
  Covered methods: 6317
  Fully covered methods: 5426
  Total methods: 7308

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Test Results

7 641 tests   7 622 ✅  2m 27s ⏱️
    7 suites     19 💤
    7 files        0 ❌

Results for commit 46be263.

♻️ This comment has been updated with latest results.