Stream SIRI-ET initial data HTTP response directly into XML parser

[vpaturet]

Summary

Stream the SIRI-ET PubSub initial data HTTP response directly into the StAX/JAXB XML parser instead of buffering the entire payload into a ByteString.

This eliminates a ~325 MB (TST) / ~580 MB (PRD) short-lived allocation that contributes to GC pressure during startup. The SiriXml.parseXml(InputStream) parser already supports streaming via XMLStreamReader, and this pattern is already used by SiriAzureUpdater and SiriFmDataSource.

Before

HTTP response → ByteString (full buffer in memory) → SiriXml.parseXml()

After

HTTP response → SiriXml.parseXml(response.body())  (streamed directly)

Unit tests

No unit tests, tested manually.

Documentation

No documentation changes needed — no new configuration options or public API changes.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.03%. Comparing base (1158ac0) to head (57914e8).
⚠️ Report is 16 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...r/google/GooglePubsubEstimatedTimetableSource.java	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #7501   +/-   ##
==========================================
  Coverage      71.02%   71.03%           
- Complexity     21002    21006    +4     
==========================================
  Files           2352     2352           
  Lines          87266    87265    -1     
  Branches        8635     8635           
==========================================
+ Hits           61983    61986    +3     
+ Misses         22293    22292    -1     
+ Partials        2990     2987    -3     

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