docs(errors): document ProxyConnectionError (UND_ERR_PRX_CONN)

[kilisamemarisaaa]

This relates to...

Follow-up documentation for #5707, which introduced ProxyConnectionError (UND_ERR_PRX_CONN) in v8.10.1. No existing issue tracks this documentation gap — found by comparing lib/core/errors.js exports and types/errors.d.ts against docs/docs/api/Errors.md.

Rationale

ProxyConnectionError is public API: it is exported from the errors namespace (lib/core/errors.js), declared in types/errors.d.ts, and thrown by lib/dispatcher/proxy-agent.js when the proxy connection is torn down so the request fails instead of retrying forever. Its sibling SecureProxyConnectionError (UND_ERR_PRX_TLS) is documented in full, but ProxyConnectionError has no class section and no row in the error-code documentation, so users cannot discover or match on a public error class that their proxies can produce at runtime.

While auditing I also verified the only other code absent from the docs table, UND_ERR_HTTP_PARSER, is intentionally different (documented as an HPE_-prefixed code on the HTTPParserError class, which extends Error directly) — so this is the single genuine documentation gap.

Changes

• Adds a ProxyConnectionError class section to docs/docs/api/Errors.md, mirroring the structure of the sibling SecureProxyConnectionError section: YAML added/changes block (v8.10.1, perf(h1): drop idle-socket timer floor with a ref'd setImmediate #5707), Extends: {UndiciError}, the name/code/cause field list matching the implementation, and the new ProxyConnectionError(cause[, message[, options]]) constructor signature matching both the implementation and the type declaration.
• Places the section directly before SecureProxyConnectionError (base class before its subclass).

Features

N/A

Bug Fixes

N/A (documentation only)

Breaking Changes and Deprecations

None

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested (S — documentation-only change; docs are not covered by the test suites)
• Benchmarked (S — not applicable)
• Documented
• Review ready
• In review
• Merge ready

[mcollina]

lgtm

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.53%. Comparing base (b73952a) to head (b44595d).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5781      +/-   ##
==========================================
+ Coverage   93.51%   93.53%   +0.01%     
==========================================
  Files         110      110              
  Lines       39359    39359              
==========================================
+ Hits        36808    36813       +5     
+ Misses       2551     2546       -5     

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