ci: make tests conditional based on changed files

Add intelligent test selection to save CI minutes:
- Added 'detect-changes' job using paths-filter to determine what changed
- Unit/integration tests only run when Python source files change
- Installer tests only run when install.sh or test_installer.sh changes

This prevents running unnecessary tests:
- Changing install.sh won't trigger Python unit/integration tests
- Changing Python source won't trigger all 5 installer tests
- Workflow changes trigger installer tests (to verify CI changes)

Saves approximately 3-4 minutes of CI time per push when only one
category of files is changed.

.github/workflows/test.yml

@@ -105,6 +105,8 @@ jobs:
   integration-tests:
     name: Integration Tests
     runs-on: ubuntu-latest
+    needs: detect-changes
+    if: needs.detect-changes.outputs.python == 'true'
 
     strategy:
       matrix:
@@ -165,6 +167,8 @@ jobs:
   installer-test:
     name: Installer Test (Fresh System)
     runs-on: ubuntu-latest
+    needs: detect-changes
+    if: needs.detect-changes.outputs.installer == 'true'
 
     strategy:
       fail-fast: false