fix: Respect caller-supplied Content-Encoding for pre-compressed request bodies

[vdusek]

A caller-supplied Content-Encoding now marks the request body as already encoded, so the client forwards it untouched instead of dropping the header (since #987) or overwriting it (before #987).

How it works

• Precedence in _prepare_request_call: caller Content-Encoding -> content-type heuristic -> size threshold -> compress. The header alone is the "hands off" signal, as in apify-client-js, so no opt-out parameter lands on the public HttpClient ABC. Lookups go through a new case-insensitive _get_header.
• The header is forwarded verbatim, so encodings the client ships no compressor for (deflate) work too, and identity becomes a per-request opt-out.
• KeyValueStoreClient.set_record gains a content_encoding argument (sync and async). Without it the behavior is barely reachable, as no public resource method took per-request headers.

Notes

• Not breaking: perf: Skip request-body compression for already-compressed content types #987 and perf: Skip request-body compression for small payloads #988 are both still in the unreleased 3.1.2 section, so the header-dropping behavior never shipped.
• Three tests from perf: Skip request-body compression for already-compressed content types #987 flip. Two collapse into one parametrized test_prepare_request_call_keeps_caller_content_encoding covering their cases plus identity and deflate.
• A Content-Encoding passed to the client constructor counts as caller-supplied on every request, as in JS, where axios config headers include the client defaults.
• The compression guide drops a claim that file-like values are streamed and never compressed. Since fix: Read file-like KVS values before upload and reject unencodable ones #965 they're read into memory and compressed like any other body.
• No integration test: the round trip needs a real token, so the API-side premise (a PUT record honors the header and serves the record with it) rests on the API docs. The unit tests cover the client side in full.

Closes #996

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.71%. Comparing base (6bd31b2) to head (f981ff2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #997      +/-   ##
==========================================
- Coverage   94.72%   94.71%   -0.02%     
==========================================
  Files          58       58              
  Lines        5332     5334       +2     
==========================================
+ Hits         5051     5052       +1     
- Misses        281      282       +1     

Flag	Coverage Δ
integration	92.05% <70.00%> (-0.28%)	⬇️
unit	84.45% <100.00%> (+<0.01%)	⬆️

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