fix(pool): propagate useH2c to connector when connections > 1

[SAY-5]

Fixes #4737.

When constructing a Pool directly (or indirectly via Agent with connections > 1), the useH2c option was not in the constructor's destructured parameter list. It ended up in ...options and was therefore not forwarded to buildConnector. The resulting connector did not set socket.alpnProtocol = 'h2' for plaintext h2c, so child Clients picked the HTTP/1 context for the new socket. The first response carrying an HTTP/2 SETTINGS frame then failed parsing with:

HTTPParserError: Response does not match the HTTP/1.1 protocol (Expected HTTP/, RTSP/ or ICE/)

connections === 1 was unaffected because Agent's default factory short-circuits to new Client(origin, opts), and Client already destructures and forwards useH2c to buildConnector.

The fix adds useH2c to Pool's destructured options, passes it to buildConnector, and stores it on kOptions for symmetry with allowH2.

I added two regression tests in test/h2c-client.js covering both Pool and Agent with useH2c: true, connections: 2 against an http2.createServer() (h2c) backend. Both tests fail on main with the parser error and pass with the fix.

Test results:

• test/h2c-client.js: 10/10 pass (8 existing + 2 new)
• test/pool.js: 35/35 pass
• test/http2-dispatcher.js: 11/11 pass
• test/http2-agent.js, test/http2-alpn.js: pass
• eslint lib/dispatcher/pool.js test/h2c-client.js: clean

Manually verified the original repro from the issue (Promise.all of 3 requests through new Agent({ useH2c: true, connections: 2 })) now succeeds.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.03%. Comparing base (0860142) to head (165a84e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5031   +/-   ##
=======================================
  Coverage   93.02%   93.03%           
=======================================
  Files         110      110           
  Lines       35793    35795    +2     
=======================================
+ Hits        33298    33303    +5     
+ Misses       2495     2492    -3     

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