Set a Wikimedia-compliant User-Agent in stream_file

[pawank925]

Fix #214

When stream_file downloads from upload.wikimedia.org, it now sets a Wikimedia-compliant User-Agent (zimscraperlib/<version> (<contact>)) unless the caller already provided one (whatever the casing). This makes scrapers compliant with the Wikimedia User-Agent policy without each of them having to remember it.

Behavior for all other hosts is left untouched, so existing callers keep sending no User-Agent by default.

Scrapers that want to advertise their own identity (e.g. myscraper/1.0 (contact) zimscraperlib/x.y.z) can still pass it through headers, like maps2zim currently does with its own wrapper. Happy to add a dedicated parameter or a module-level configuration if you'd prefer that instead of relying on headers.

Changes:

• constants.py: new WIKIMEDIA_UPLOAD_HOST constant.
• download.py: two small helpers (is_wikimedia_upload_url, has_user_agent_header) and the default User-Agent applied for upload.wikimedia.org.
• tests: unit tests for the helpers and for the stream_file behavior (default UA applied, custom UA preserved, extra headers merged).
• CHANGELOG entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (93709f8) to head (ad49f2c).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #353   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           42        42           
  Lines         2720      2728    +8     
  Branches       404       405    +1     
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+ Hits          2720      2728    +8     

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[benoit74]

LGTM