feat(client): make the WeakClient public and introduce Client::downgrade() to make it easier to get one

[bnjbvr]

The WeakClient (weak reference to a Client) is actually useful for external embedders too, who'd like to have access to Clients in long-running background tasks, but that shouldn't maintain the Client alive in this case.

First commit makes it public. Second commit mimick's Arc's API and introduces Client::downgrade(), which creates a WeakClient, and seems less weird / more idiomatic than the previous WeakClient::from_client method.

• I've documented the public API changes in the appropriate changelog files (see [Writing changelog entries][pull-request-template-changelog]).
• This PR was made with the help of AI.
  • all previous usages of WeakClient::from_client have been replaced with AI. I've reviewed those.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (e3a21fd) to head (72599ce).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7244      +/-   ##
==========================================
- Coverage   90.43%   90.41%   -0.02%     
==========================================
  Files         416      416              
  Lines      122930   122930              
  Branches   122930   122930              
==========================================
- Hits       111174   111152      -22     
- Misses       7617     7626       +9     
- Partials     4139     4152      +13     

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[codspeed]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing nordeck:public-weak-client (72599ce) with main (e3a21fd)}