ci: ignore browser assets in license lint

Merge #5612

## Summary
- Added `.github/header-checker-lint.yml` with full configuration to ignore `src/google/adk/cli/browser/**`.
- Ensured Python files are still checked by adding `py` to `sourceFileExtensions`

## Test Plan
- Verify that the `License Header Lint GCF` / `header-check` status check passes on this PR

Co-authored-by: Wei Sun (Jack) <weisun@google.com>
COPYBARA_INTEGRATE_REVIEW=#5612 from google:ci/ignore-browser-assets d08d161
PiperOrigin-RevId: 911583387

Author: Jacksunwei

.github/header-checker-lint.yml

@@ -0,0 +1,13 @@
+allowedCopyrightHolders:
+  - 'Google LLC'
+allowedLicenses:
+  - 'Apache-2.0'
+  - 'MIT'
+  - 'BSD-3'
+sourceFileExtensions:
+  - 'ts'
+  - 'js'
+  - 'java'
+  - 'py'
+ignoreFiles:
+  - 'src/google/adk/cli/browser/**'