feat: Unify the request pipeline across HTTP clients - #1011

Every `impit.HTTPError` counted as transient, so a transport failure a
retry cannot fix burned the whole backoff before surfacing. The
permanent classes now fail on the first attempt instead:

- `LocalProtocolError` - a request Impit rejects before sending it, e.g.
one carrying an invalid header value.
- `TooManyRedirects` - a routing loop, which repeating the request
cannot break.
- `UnsupportedProtocol` - the class Impit declares for a scheme it
refuses to speak, but does not raise today; an unsupported scheme
arrives as `impit.InvalidURL`, outside the `impit.HTTPError` tree and
non-retryable already. Listed so the classifier stays right if Impit
switches over.
- `HTTPStatusError` - only `Response.raise_for_status()` raises it and
the client never calls it; `_make_request` decides on status codes from
the response itself.

Everything else in the `impit.HTTPError` tree stays retryable, including
a bare `HTTPError` - Impit wraps a failure its internal HTTP library did
not classify in one, e.g. a non-HTTP response or a connection reset -
and a `ProxyError`, which covers a rejected CONNECT tunnel and a 407
alike, so a transient case cannot be told from a permanent one.

Also drops `InvalidResponseBodyError` from the retry check. Only
`key_value_store.py` raises it, once `call` has already returned, so it
can never reach the classifier. Its docstring no longer claims the
client retries such requests. The duplicate `is_retryable_error` in
`_utils/errors.py`, which carried the old policy, is gone.

Split out of #1006 and re-targeted at `master`, so the retry fixes can
ship in a patch release ahead of the pipeline refactor in #1011.

*✍️ Drafted by Claude Code*

…1020)

Before raising `ApifyApiError`, `_make_request` buffers the response so
a streamed error body reaches the exception. That read sat outside the
`try` block that classifies an attempt, so a failure there escaped the
retry policy entirely: a non-retryable failure was retried through the
whole backoff, and the response was left open.

The read now goes through the same classification as a failed send —
retried when it is a transient transport failure, surfaced immediately
when it is not — and the response is closed before the exception
propagates.

Split out of #1006 and re-targeted at `master`, so the retry fixes can
ship in a patch release ahead of the pipeline refactor in #1011.

*✍️ Drafted by Claude Code*

Moves `call`, the retry loop, and the per-attempt request handling from
`ImpitHttpClient` and `ImpitHttpClientAsync` into `HttpClient` and
`HttpClientAsync`, leaving Impit as a thin adapter over the transport
hooks: `send_request`, `is_retryable_transport_error`,
`is_timeout_error`, and `close()` / `aclose()` plus context managers. A
custom client that overrides `call` keeps working unchanged and opts out
of the shared pipeline. `StreamedLog` classifies timeouts through the
transport-neutral `is_timeout_error` hook instead of importing Impit.

No behavior change for the built-in clients: the retry policy shipped in
#1019 (permanent transport errors fail fast) and #1020 (a failed read of
a streamed error body is classified like a failed send and the response
is closed) is preserved — the classification now lives in the
`is_retryable_transport_error` hook, and the error-body read handling in
the shared pipeline.

Supersedes #1011, rebased onto current master and squashed. Originally
split out of #1006.

*✍️ Drafted by Claude Code*