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.

[rohityan]

Hi @MukundaKatta, Thank you for your contribution through this pull request! This PR has merge conflicts that require changes from your end. Could you please rebase your branch with the latest main branch to address these? Once this is complete, please let us know so we can proceed with the review.