client: don't reuse a pick's authority override on a later attempt

[nvxbug]

csAttempt.newStream writes the gRFC A81 authority override from the pick result into cs.callHdr, which is allocated once per clientStream and shared by every attempt. Nothing clears Authority between attempts, so once an attempt is picked to an endpoint that carries a hostname, a retry whose own pick supplies no override still goes out with the earlier endpoint's authority. An xDS cluster using auto_host_rewrite whose endpoints do not all set hostname hits this on any retry, and :authority is what selects the virtual host and what RBAC host matchers run against on the receiving side.

Copy the call header per attempt so the override, like PreviousAttempts, stays with the attempt that produced it. The value is derived from that attempt's pick result, so the attempt is the right scope for it; leaving it on the stream lets one endpoint's rewrite outlive the pick that asked for it.

RELEASE NOTES:

• client: Fix a bug where a retried RPC could be sent with the :authority value belonging to the endpoint chosen by an earlier attempt, when a load balancer supplies an authority override (e.g. xDS auto_host_rewrite)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.08%. Comparing base (89d4d61) to head (ec538d8).
⚠️ Report is 28 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9277      +/-   ##
==========================================
- Coverage   83.14%   83.08%   -0.07%     
==========================================
  Files         422      423       +1     
  Lines       34862    35322     +460     
==========================================
+ Hits        28985    29346     +361     
- Misses       4385     4451      +66     
- Partials     1492     1525      +33     

Files with missing lines	Coverage Δ
stream.go	82.52% <100.00%> (-0.17%)	⬇️

... and 52 files with indirect coverage changes

🚀 New features to boost your workflow:

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

[Pranjali-2501]

Hi @nvxbug , Thank you for raising a PR.

I have added some comments, PTAL.

[easwars]

@nvxbug : Thanks for your contribution!