transport: prevent nil pointer dereference in recvBuffer.put.

[Pranjali-2501]

When b.err is already set on a recvBuffer, put() drops every later recvMsg and frees its buffer by calling r.buffer.Free(). A recvMsg that carries only an error or io.EOF has no payload, so its buffer is nil, and the Free() call panics with a nil pointer dereference.

This change adds a nil check before calling r.buffer.Free() and adds an end-to-end test covering this scenario.

RELEASE NOTES:

• transport: Fix a server panic (nil pointer dereference) that a client could trigger by sending a second DATA frame with END_STREAM on a stream whose RPC handler had already returned.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.81%. Comparing base (529d6e8) to head (333ae56).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9533      +/-   ##
==========================================
+ Coverage   87.76%   87.81%   +0.04%     
==========================================
  Files         437      437              
  Lines       31406    31407       +1     
==========================================
+ Hits        27565    27579      +14     
+ Misses       3841     3828      -13     

Files with missing lines	Coverage Δ
internal/transport/transport.go	94.05% <100.00%> (+0.03%)	⬆️

... and 19 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.