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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