[build-tools] handle custom Gymfile err in xcactivitylog

[hSATAC]

Why

Fix Sentry TURTLE-V2-WORKER-1WT (ENG-21383) — 48 events of xclogparser failing during the PARSE_XCACTIVITYLOG phase with stderr:

Error: The file "Build" couldn't be opened because there is no such file.

runFastlaneGym always returns derivedDataPath = <ios>/build, but projects with a custom ios/Gymfile can set a different derived_data_path (or omit it entirely, in which case Xcode uses its default DerivedData location). xclogparser then runs against a directory that has no .xcactivitylog files and fails.

How

Added a pre-flight check in parseAndReportXcactivitylog that lists <derivedDataPath>/Logs/Build/:

• If no .xcactivitylog files are present, log actionable guidance pointing users at the Gymfile lines they need and skip the analysis (no Sentry — this is user config, not an engineering bug).
• If readdir fails with anything other than ENOENT (e.g. EACCES, EMFILE), rethrow so it still lands in Sentry under a new checking_xcactivitylog_existence phase tag — real infra problems are not masked by the swallow.

Return type changed from Promise<void> to Promise<{ skipped: boolean }> so the caller in builders/ios.ts can call ctx.markBuildPhaseSkipped() and reflect the correct phase status in the build summary.

Test Plan

Unit tests in xcactivitylog.test.ts cover the four pre-check branches:

• Logs/Build exists but has no .xcactivitylog files → skipped + actionable Gymfile log, no Sentry
• Logs/Build does not exist (ENOENT) → skipped, no Sentry
• Logs/Build has only non-.xcactivitylog entries (e.g. LogStoreManifest.plist) → skipped
• readdir fails with EACCES → Sentry captured with checking_xcactivitylog_existence phase tag, no misleading Gymfile message logged

Existing tests updated to assert the new { skipped } return shape. 41/41 tests pass; typecheck/lint/fmt clean.

[linear]

ENG-21383

[github-actions]

⏩ The changelog entry check has been skipped since the "no changelog" label is present.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.22%. Comparing base (149f5d8) to head (f9eb7f4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3790      +/-   ##
==========================================
+ Coverage   57.21%   57.22%   +0.02%     
==========================================
  Files         905      905              
  Lines       39290    39303      +13     
  Branches     8235     8237       +2     
==========================================
+ Hits        22475    22488      +13     
  Misses      15345    15345              
  Partials     1470     1470              

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[github-actions]

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