perf(proxy-agent): avoid extra header allocations in auth guard

[trivikr]

This relates to...

ProxyAgent request header handling performance.

Rationale

ProxyAgent does extra work on in it's Proxy-Authorization guard every dispatch. Object headers currently allocate an Object.keys() array before scanning, and array headers are converted before the guard runs.

Changes

• Check array headers for Proxy-Authorization while converting them in buildHeaders().
• Replace Object.keys().find() with a for...in early-exit scan for object headers.

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

❌ Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.26%. Comparing base (584e6ae) to head (dd0c2b1).

Files with missing lines	Patch %	Lines
lib/dispatcher/proxy-agent.js	80.95%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5164      +/-   ##
==========================================
- Coverage   93.26%   93.26%   -0.01%     
==========================================
  Files         110      110              
  Lines       36353    36370      +17     
==========================================
+ Hits        33906    33919      +13     
- Misses       2447     2451       +4     

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[mcollina]

lgtm