Improve changed app detection in CI (#37838)

This pull request improves the robustness and reliability of the script
and workflows that detect changed or new maintained apps in pull
requests. The main focus is on making the detection script pass
validation when the test is triggered but no new FMAs are detected.

**Script robustness and error handling:**

* The `.github/scripts/detect-new-fmas-in-pr.sh` script is updated to
always exit successfully (status 0) when no changes are detected, and
only exit with error (status 1) for critical failures like missing `jq`.
A new `safe_exit` function is introduced to standardize output and
ensure graceful exits.
[[1]](diffhunk://#diff-f9bbb0340f504713c99d610f3c64bf281fc13ed3cb8a1c06a5366272c9828a8dR7-R11)
[[2]](diffhunk://#diff-f9bbb0340f504713c99d610f3c64bf281fc13ed3cb8a1c06a5366272c9828a8dL21-R39)
* Improved error handling for missing files, empty variables, and failed
commands throughout the script, including handling cases where
`merge-base`, `git show`, or `jq` fail, and ensuring empty or missing
data does not cause the script to error out.
[[1]](diffhunk://#diff-f9bbb0340f504713c99d610f3c64bf281fc13ed3cb8a1c06a5366272c9828a8dL32-R66)
[[2]](diffhunk://#diff-f9bbb0340f504713c99d610f3c64bf281fc13ed3cb8a1c06a5366272c9828a8dR87-R108)
[[3]](diffhunk://#diff-f9bbb0340f504713c99d610f3c64bf281fc13ed3cb8a1c06a5366272c9828a8dL75-R155)

**Workflow improvements:**

* The `test-fma-darwin-pr-only.yml` and `test-fma-windows-pr-only.yml`
workflows are updated to default to "no changes" if the detection step
fails or does not set the expected output, preventing false positives or
workflow failures.
[[1]](diffhunk://#diff-28b30c8601cb7662d59efbfbbcf800cae91455fd3d875627659dced8c1257a24L70-R72)
[[2]](diffhunk://#diff-51641fd1d2cc19348b81fd8310b62ad270ca5082ceddff2d49064e78f126a1eaL76-R78)

.github/workflows/test-fma-darwin-pr-only.yml

@@ -67,7 +67,9 @@ jobs:
       - name: Check if there are changes
         id: check-changes
         run: |
-          if [ "${{ steps.detect-changed.outputs.HAS_CHANGES }}" == "true" ]; then
+          # Default to no changes if detection step failed or didn't set output
+          HAS_CHANGES="${{ steps.detect-changed.outputs.HAS_CHANGES }}"
+          if [ "$HAS_CHANGES" == "true" ]; then
             echo "has_changes=true" >> $GITHUB_OUTPUT
             echo "Changed apps detected: ${{ steps.detect-changed.outputs.CHANGED_APPS }}"
           else

.github/workflows/test-fma-windows-pr-only.yml

@@ -73,7 +73,9 @@ jobs:
       - name: Check if there are changes
         id: check-changes
         run: |
-          if ("${{ steps.detect-changed.outputs.HAS_CHANGES }}" -eq "true") {
+          # Default to no changes if detection step failed or didn't set output
+          $hasChanges = "${{ steps.detect-changed.outputs.HAS_CHANGES }}"
+          if ($hasChanges -eq "true") {
             "has_changes=true" | Out-File -FilePath $env:GITHUB_OUTPUT -Encoding utf8 -Append
             Write-Host "Changed apps detected: ${{ steps.detect-changed.outputs.CHANGED_APPS }}"
           } else {