Add retry_request / aretry_request helpers for body-phase errors

[will-ockmore]

feat: add retry_request / aretry_request helpers for body-phase errors

HTTPX transports return once response headers arrive, so errors raised while reading the body (ReadTimeout, truncated RemoteProtocolError) bypass RetryTransport and are never retried. These helpers run the retry loop at the client level, where the body is read, closing that gap with the same Retry configuration.

• Extract the retry loop to module-level _retry_operation(_async) shared by RetryTransport and the helpers.
• retry_request / aretry_request: retry header-phase errors, retryable status codes, and body-phase errors for buffered (non-streaming) requests.
• Honour Retry.allowed_methods (idempotent-only by default); forward auth / follow_redirects to client.send; raise if the client already uses RetryTransport (would retry twice).
• Docs: FAQ explains the niche and when to prefer RetryTransport; README pointer and API docstrings link to it.

Additionally, backfill CHANGELOG for the helpers, validate_response, request.extensions["retry"], and Retry.copy_with.

Closes #29

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e9c1c5d) to head (ea032d2).

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #79    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            6         8     +2     
  Lines         1220      1460   +240     
==========================================
+ Hits          1220      1460   +240     

Files with missing lines	Coverage Δ
httpx_retries/__init__.py	100.00% <100.00%> (ø)
httpx_retries/helpers.py	100.00% <100.00%> (ø)
httpx_retries/transport.py	100.00% <100.00%> (ø)
tests/test_helpers.py	100.00% <100.00%> (ø)
tests/test_transport.py	100.00% <100.00%> (ø)

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