impl(showcase): regenerate showcase to use new streaming constructors

[suzmue]

The integration test also needs to be updated due to the eager initialization changes.

For #2318

[gemini-code-assist]

Code Review

This pull request refactors the bidirectional streaming APIs (chat and connect) in the generated showcase client. It replaces the build method on request builders with a send method returning a Result, introduces fluent request setter methods, and updates the underlying stub, tracing, and transport layers to accept the initial request parameter and return a Result. The transport implementation has also been simplified to use RequestSender::from_fn and ResponseReceiver::from_stream instead of spawning a background task. I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.27%. Comparing base (516e38e) to head (e95340b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6310   +/-   ##
=======================================
  Coverage   96.27%   96.27%           
=======================================
  Files         283      283           
  Lines       73110    73110           
=======================================
  Hits        70384    70384           
  Misses       2726     2726           

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