fix: Fail fast on transport errors a retry cannot fix

[vdusek]

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.85%. Comparing base (334e131) to head (fbc7216).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1019      +/-   ##
==========================================
- Coverage   94.87%   94.85%   -0.03%     
==========================================
  Files          58       58              
  Lines        5367     5365       -2     
==========================================
- Hits         5092     5089       -3     
- Misses        275      276       +1     

Flag	Coverage Δ
integration	91.94% <75.00%> (-0.27%)	⬇️
unit	85.14% <100.00%> (-0.01%)	⬇️

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