docs: fix multiple inaccuracies in API documentation

[AliMahmoudDev]

Fixes #5382

Changes

GettingStarted.md

• Error handling example used errors.ClassName.code (static property) but .code is only set as an instance property in the constructor — it evaluates to undefined and the switch/case branches never match. Changed to string literals ("UND_ERR_CONNECT_TIMEOUT", etc.)

RetryHandler.md

• Fixed constructor signature: documented 3 params (dispatchOptions, retryHandlers, [retryOptions]) but code takes 2 (opts, { dispatch, handler }) with retryOptions nested inside opts
• Added missing TRACE to default methods array
• Added missing EHOSTUNREACH and EPIPE to default errorCodes array

RetryAgent.md

• Fixed return type: ProxyAgent → RetryAgent
• Same methods and errorCodes defaults fixes as RetryHandler

BalancedPool.md

• Fixed property heading: Pool.stats → BalancedPool.stats

Errors.md

• Fixed Socks5ProxyError error code: UND_ERR_SOCKS5* (wildcard) → UND_ERR_SOCKS5

MockAgent.md

• Fixed method name in example: clearAllCallHistory() → clearCallHistory()

Dispatcher.md

• Fixed broken internal link: Dispatch.md → Dispatcher.md
• Added missing onBodySent callback to DispatchHandler interface documentation

All changes verified against the actual source code in lib/.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.25%. Comparing base (8464ab7) to head (c435c7e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5384   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files         110      110           
  Lines       36752    36752           
=======================================
  Hits        34274    34274           
  Misses       2478     2478           

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