docs: point event_hooks at a caller-built httpx2 client

[lesnik512]

Problem

docs/middleware.md recommends event_hooks "on the wrapped httpx2_client" for low-level transforms. Since 0.18.0 (#148), httpware forwards every httpx2 client option except event_hooks, which it refuses. The guide didn't say why, so the two read as contradictory.

Change

The bullet now says to put hooks on an httpx2 client you build and pass as httpx2_client=, and explains the refusal. An httpx2.HTTPStatusError raised in a hook (for example by raise_for_status()) reaches the chain as a plain TransportError, not a StatusError. AsyncRetry only retries StatusError/NetworkError/TimeoutError, and the circuit breaker only counts those, so the error is neither retried nor counted.

Verification

Checked by hand: a hook calling raise_for_status() on a 503, behind AsyncRetry, raises TransportError after one attempt. mkdocs build --strict passes.