Fix Triple gRPC decoder handoff

[Anon2Tokyo]

What is the purpose of the change?

Fixes #16414.

This change fixes the gRPC no-stub method discovery path for Triple streaming requests. The lazy method discovery listener now reuses the stream's existing StreamingDecoder instead of creating a temporary GrpcStreamingDecoder, so buffered bytes from a subsequent message are preserved when the listener switches to the resolved business handler.

The close callback on the temporary method-discovery listener is also made a no-op because it only exists to resolve the method descriptor and should not close the actual request stream.

A regression test covers a DATA-frame layout where the first decode contains one complete gRPC message followed by the prefix of the next message, then verifies the second message is still decoded after the listener switch.

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

Test:

mvn.cmd -pl dubbo-rpc/dubbo-rpc-triple -Dtest=GrpcStreamingDecoderTest test

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.90%. Comparing base (3a30432) to head (d4ea6b3).

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16416      +/-   ##
============================================
+ Coverage     60.87%   60.90%   +0.02%     
  Complexity    11766    11766              
============================================
  Files          1953     1953              
  Lines         89273    89272       -1     
  Branches      13473    13473              
============================================
+ Hits          54346    54369      +23     
+ Misses        29333    29314      -19     
+ Partials       5594     5589       -5     

Flag	Coverage Δ
integration-tests-java21	32.08% <100.00%> (-0.06%)	⬇️
integration-tests-java8	32.21% <100.00%> (+<0.01%)	⬆️
samples-tests-java21	32.17% <0.00%> (-0.03%)	⬇️
samples-tests-java8	29.81% <0.00%> (+0.02%)	⬆️
unit-tests-java11	59.11% <100.00%> (-0.01%)	⬇️
unit-tests-java17	58.64% <100.00%> (-0.01%)	⬇️
unit-tests-java21	58.61% <100.00%> (+<0.01%)	⬆️
unit-tests-java25	58.56% <100.00%> (+<0.01%)	⬆️
unit-tests-java8	59.14% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.