fix(pool): add useH2c option support for multiple connections

[rambo-panda]

This relates to...

#4737

Rationale

The Pool class was not passing the useH2c option to buildConnector and kOptions, causing HTTPParserError when using Agent with useH2c and connections > 1.

Changes

This fix ensures that:

• useH2c is extracted from constructor parameters
• useH2c is passed to buildConnector for proper connection setup
• useH2c is included in kOptions for client instances

Features

N/A

Bug Fixes

Fixes issue with h2c (HTTP/2 Cleartext) failing on second connection when Pool creates multiple Client instances.

Breaking Changes and Deprecations

Before: connections > 1 with useH2c caused HTTP/1.1 parser to receive
HTTP/2 binary frames, resulting in protocol mismatch error
After: All connections properly use HTTP/2 cleartext protocol

Status

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

[mcollina]

Thanks for opening a PR! Can you please add a unit test?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.88%. Comparing base (250efc8) to head (c0df7b2).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4738      +/-   ##
==========================================
- Coverage   92.89%   92.88%   -0.01%     
==========================================
  Files         109      109              
  Lines       33886    33888       +2     
==========================================
  Hits        31478    31478              
- Misses       2408     2410       +2     

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