docs: fix code examples that crash at runtime and other inaccuracies

[AliMahmoudDev]

Fixes #5385

High severity

Dispatcher.md — DNS interceptor examples missing Agent import
Both DNS examples import Client but use new Agent(). Running these gives ReferenceError: Agent is not defined. Changed import to Agent.

H2CClient.md — await in non-async .then() callback
The example uses await inside .then(() => { which is a SyntaxError. Added async to the callback.

EnvHttpProxyAgent.md — Destructuring json from Response
Two examples destructure json from a fetch Response (const { status, json } = await fetch(...)), but json is a prototype method — destructuring yields undefined and await json() throws TypeError. Changed to use response.json().

Medium severity

Dispatcher.md — Retry example heading says "Redirect" (copy-paste)
Changed to "Basic Retry Interceptor".

Cookies.md — sameSite type says "String" not "Strict"

RetryHandler.md — retry callback return type wrong
Changed number | null to void to match implementation and RetryAgent.md.

Low severity

Fetch.md — Heading says Header not Headers

All changes verified against the actual source code.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.25%. Comparing base (8464ab7) to head (986b4cd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5386   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files         110      110           
  Lines       36752    36752           
=======================================
  Hits        34274    34274           
  Misses       2478     2478           

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[mcollina]

lgtm