fix(web): restore build_graph_image endpoint

[MukundaKatta]

Summary

• restore the legacy /dev/build_graph_image/{app_name} route that the bundled dev UI still calls
• render the existing agent graph as PNG bytes through the backend alias instead of returning 404
• add a regression test that verifies the endpoint returns image/png and forwards dark_mode correctly

Testing

• python3 -m py_compile src/google/adk/cli/adk_web_server.py tests/unittests/cli/test_fast_api.py
• PYTHONPATH=src python3 -m pytest tests/unittests/cli/test_fast_api.py -k build_graph_image_returns_png_bytes (fails locally: ModuleNotFoundError: No module named 'google.genai')

Closes #5430

[adk-bot]

Response from ADK Triaging Agent

Hello @MukundaKatta, thank you for creating this PR!

This PR is a bug fix. Could you please associate a GitHub issue with this PR? If there is no existing issue, could you please create one?

This information will help reviewers to review your PR more efficiently. Thanks!

[MukundaKatta]

Linked #5430 (the bug report I opened for this). Updated the PR body so the triage agent can pick it up.