fix: Propagate API token to custom HTTP clients

[vdusek]

ApifyClient.with_custom_http_client(token=...) (and the async twin) stored the token on the ApifyClient instance but never passed it to the injected HTTP client, so no request carried an Authorization header and every call failed with 401. The documented custom HTTP client example inherited the bug.

• with_custom_http_client now sets Authorization: Bearer <token> on the injected client's default headers, unless the client already has an auth header configured (checked case-insensitively).
• HttpClientBase._prepare_request_call now merges the client's default headers under the per-request headers, so any custom client using the helper (including a pre-built ImpitHttpClient passed as the custom client) actually sends them. For the default client the wire behavior is unchanged, since impit request-level headers replace the identical client-level ones.
• The HTTPX guide examples now merge self._headers before delegating, and the HttpClient ABC docstring states that implementations must send the default headers with every request.

Regression tests cover the token reaching the wire (custom sync/async clients and a pre-built tokenless ImpitHttpClient) and the no-clobber semantics for client-configured auth headers.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.65%. Comparing base (a8a393b) to head (d96eb7f).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #956      +/-   ##
==========================================
+ Coverage   94.61%   94.65%   +0.03%     
==========================================
  Files          58       58              
  Lines        5239     5257      +18     
==========================================
+ Hits         4957     4976      +19     
+ Misses        282      281       -1     

Flag	Coverage Δ
integration	92.46% <65.00%> (-0.21%)	⬇️
unit	83.92% <100.00%> (+0.18%)	⬆️

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[Pijukatel]

Impit client was working without this, so I guess this is going to duplicate header handling at this level for the default Impit client.

https://github.com/apify/impit/blob/accab5669aa3c3d8e5dc3adc75c847294eabd104/impit/src/impit.rs#L413

[vdusek]

It won't duplicate the headers because we use _prepare_request_call, where we do deduplication. But yeah, it would make sense to update the Impit init, stop passing the default headers, and rely on the upstream behavior.

[vdusek]

I'll also update it to handle the fact, that HTTP headers should be case-insensitive (according to RFC 9110). And also add more tests regarding the headers.