fix: keep multi-step auth responses under the response body cap

[lesnik512]

httpx2's _send_handling_auth reads every intermediate auth-flow response with an unbounded response.read(). Any auth= that takes more than one step therefore bypassed max_response_body_bytes, with or without redirects: the 401 challenge of httpx2.DigestAuth, and custom flows such as a token refresh that retries after calling a token endpoint. auth= has been forwarded since 0.18, so a server could make a capped client buffer an arbitrarily large challenge body.

How

With a cap set, httpware now drives the auth flow itself (auth.async_auth_flow / sync_auth_flow, public API), wrapped around the redirect loop from #152, the same nesting httpx2 uses.

• Every wire request goes out with a no-op httpx2.Auth(), so httpx2 never applies auth on its own. Which auth runs is decided like httpx2 does: client.auth, else Basic from URL credentials, else none (_request_auth, a public-API copy of the private _build_request_auth).
• Intermediate auth responses are closed unread and kept in history. An auth with requires_response_body = True gets each response buffered under the cap instead, so an oversized token response raises ResponseTooLargeError.
• Behaviour otherwise matches the no-cap path, checked against native httpx2: auth steps count toward max_redirects (checked before every send), nested .history of each hop, history order across auth steps, and the error raised for a malformed Digest challenge.
• Without a cap nothing changes.

Side effects

• DigestAuth now answers a challenge that arrives after a redirect under a cap. In 0.20.0 it could not (noted in that release).
• Credentials embedded in a redirect Location are no longer applied on the capped path, matching httpx2 (0.20.0 sent hops with auth=None, which applied them).
• stream() with a requires_response_body auth yields a body already read under the cap. httpx2 reads that body anyway; docs/errors.md now says so.
• _read_capped_and_close[_async] is shared by _terminal and the auth loop.

Tests

tests/test_client_body_cap_auth.py, sync and async mirrored: Digest challenge body read only without a cap (verbs and stream()), token refresh under and over the cap (verbs and stream()), URL credentials with and without a cap, Digest after a redirect with identical nested history with and without a cap, auth steps counting toward max_redirects, and a malformed challenge raising the same TransportError. The Digest and stream tests fail against main.

Verification

just lint-ci, just test-ci (1001 passed, 100% coverage), just docs-build, just adr-check.