chore(http2): collapse duplicate request stream setup

[trivikr]

This relates to...

Internal HTTP/2 client request stream setup cleanup.

Rationale

The expectContinue and normal request paths in client-h2 duplicated stream creation and setup.
Sharing that setup reduces branch/code overhead and keeps the hot path easier to tune later.

Changes

• Set the 100-continue header only when expectContinue is enabled.
• Use one shared requestStream creation and setup path for both continue and normal requests.
• Preserve the existing continue-specific listener and body write behavior.

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

Assisted-by: openai:gpt-5.5

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.25%. Comparing base (0ca054a) to head (68cc047).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5140   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files         110      110           
  Lines       36249    36241    -8     
=======================================
- Hits        33803    33797    -6     
+ Misses       2446     2444    -2     

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