[Java] Honor GOOGLE_CLOUD_UNIVERSE_DOMAIN for the Dataflow endpoint

[shunping]

Part of #40569, Builds upon #40670 (needs universe domain support from googleapis/google-api-java-client#2435)

Problem

The Java Dataflow runner always sends requests to https://dataflow.googleapis.com/, even when GOOGLE_CLOUD_UNIVERSE_DOMAIN points to another universe.
apiRootUrl defaults to https://dataflow.googleapis.com/, and DataflowTransport always calls Dataflow.Builder#setRootUrl. In google-api-client, calling setRootUrl (even with the default URL) marks the endpoint as user-configured, so the library never builds the https://dataflow.<universe>/ endpoint.

Changes

• apiRootUrl no longer has a default.
• DataflowTransport sets the root URL and service path only when apiRootUrl or dataflowEndpoint is set. Otherwise the client library resolves the endpoint, honoring GOOGLE_CLOUD_UNIVERSE_DOMAIN and falling back to the public URL.

dataflowEndpoint	apiRootUrl	Endpoint
unset	unset	resolved by client library (universe-aware)
relative path	unset	universe-aware root + path
relative path / unset	set	apiRootUrl (+ path)
absolute URL	any	dataflowEndpoint

Breaking change

DataflowPipelineDebugOptions.getApiRootUrl() now returns null by default instead of https://dataflow.googleapis.com/.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (76f6e6d) to head (a6f8095).
⚠️ Report is 247 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40672       +/-   ##
=============================================
- Coverage     56.16%        0   -56.17%     
=============================================
  Files          1124        0     -1124     
  Lines        178336        0   -178336     
  Branches       1489        0     -1489     
=============================================
- Hits         100170        0   -100170     
+ Misses        75645        0    -75645     
+ Partials       2521        0     -2521     

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[shunping]

/gemini review

[github-actions]

📋 Review Summary

This PR updates the Java Dataflow runner to defer Dataflow API root URL resolution to the Google API client library when neither --apiRootUrl nor an absolute --dataflowEndpoint is explicitly configured, enabling GOOGLE_CLOUD_UNIVERSE_DOMAIN support. The implementation, nullability annotations, documentation updates, and unit test coverage in DataflowTransportTest are clean and well-structured.

🔍 General Feedback

• Removing @Default.String(Dataflow.DEFAULT_ROOT_URL) from DataflowPipelineDebugOptions#getApiRootUrl() and conditionally calling builder.setRootUrl(...) in DataflowTransport accurately avoids setting isUserConfiguredEndpoint = true in AbstractGoogleClient.Builder, preserving universe domain resolution.
• The subtle behavior change for callers inspecting getApiRootUrl() (now returning null when unset) is appropriately annotated with @Nullable and documented in CHANGES.md.

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[github-actions]

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