chore: run library generation jobs conditionally (#2698)

In this PR:
- Add a job to get changed directories in pull request.
- Run library generation tests only if the changed directories contains
`library_generation`.

Verified in
- [Run
tests](https://github.com/googleapis/sdk-platform-java/actions/runs/8887270660/job/24402267953)
- [Don't run
tests](https://github.com/googleapis/sdk-platform-java/actions/runs/8887279933/job/24402294618)

The goal is to make library generation tests as a required check.
Moreover, we don't want these tests run against every pull requests
because only pull requests change library_generation directory should be
tested.

Currently, the workflow can only run if the files changed are within
`paths`. Therefore, we can't set this workflow as required because it
will get stuck and prevent merging for pull requests that don't change
files within paths.

Using this approach, this workflow runs on every pull request and only
run actual tests when the condition is met (determined by the first
job).

Official docs:
https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/collaborating-on-repositories-with-code-quality-features/troubleshooting-required-status-checks#handling-skipped-but-required-checks

We can make the workflow required after merging this PR and the tests in
the workflow will only run if the changed directories contains
`library_generation`.

.github/workflows/verify_library_generation.yaml

@@ -3,14 +3,38 @@ on:
     branches:
     - main
   pull_request:
-    paths:
-    - library_generation/**
 
   workflow_dispatch:
 name: verify_library_generation
 jobs:
-  integration_tests:
+  should-run-library-generation-tests:
     runs-on: ubuntu-22.04
+    outputs:
+      should_run: ${{ steps.get_changed_directories.outputs.should_run }}
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+    - name: get changed directories in the pull request
+      id: get_changed_directories
+      shell: bash
+      run: |
+        set -ex
+        git checkout "${base_ref}"
+        git checkout "${head_ref}"
+        changed_directories="$(git diff --name-only ${base_ref} ${head_ref})"
+        if [[ ${changed_directories} =~ "library_generation/" ]]; then
+          echo "should_run=true" >> $GITHUB_OUTPUT
+        else
+          echo "should_run=false" >> $GITHUB_OUTPUT
+        fi
+      env:
+        base_ref: ${{ github.event.pull_request.base.ref }}
+        head_ref: ${{ github.event.pull_request.head.ref }}
+  library-generation-integration-tests:
+    runs-on: ubuntu-22.04
+    needs: should-run-library-generation-tests
+    if: needs.should-run-library-generation-tests.outputs.should_run == 'true'
     steps:
     - uses: actions/checkout@v4
     - uses: actions/setup-python@v5
@@ -41,12 +65,14 @@ jobs:
       run: |
         set -x
         python -m unittest library_generation/test/integration_tests.py
-  unit_tests:
+  library-generation-unit-tests:
+    runs-on: ${{ matrix.os }}
+    needs: should-run-library-generation-tests
+    if: needs.should-run-library-generation-tests.outputs.should_run == 'true'
     strategy:
       matrix:
         java: [ 8 ]
         os: [ ubuntu-22.04, macos-12 ]
-    runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4
     - name: install utils (macos)
@@ -88,8 +114,10 @@ jobs:
       run: |
         set -x
         python -m unittest discover -s library_generation/test/ -p "*unit_tests.py"
-  lint-shell:
+  library-generation-lint-shell:
     runs-on: ubuntu-22.04
+    needs: should-run-library-generation-tests
+    if: needs.should-run-library-generation-tests.outputs.should_run == 'true'
     steps:
     - uses: actions/checkout@v4
     - name: Run ShellCheck
@@ -98,8 +126,10 @@ jobs:
         scandir: 'library_generation'
         format: tty
         severity: error
-  lint-python:
+  library-generation-lint-python:
     runs-on: ubuntu-22.04
+    needs: should-run-library-generation-tests
+    if: needs.should-run-library-generation-tests.outputs.should_run == 'true'
     steps:
     - uses: actions/checkout@v4
     - name: install python dependencies