repository: drop "wait" arg, make put/delete synchronous

[ThomasWaldmann]

What

The new borgstore-based Repository still carried put(..., wait=) / delete(..., wait=) and an async_response() method, inherited from the legacy RemoteRepository's pipelined RPC protocol.

In the new architecture these are dead:

• borgstore's Store/backend API is strictly synchronous, so put/delete ignored wait and always ran synchronously;
• async_response() was an empty stub that always returned None.

This PR removes wait and async_response() so the synchronous behavior is explicit, and cleans up every caller that still threaded wait=False / drained async_response():

• Repository.put / Repository.delete — drop wait; remove async_response().
• Cache.add_chunk — drop wait, call repository.put(id, cdata).
• archive.py — drop wait=False from the three add_chunk calls, remove the per-call async_response(wait=False) and the while ... async_response(wait=True) drain loop in save().
• archiver/transfer_cmd.py — drop wait=False from the add_chunk calls and the async_response(wait=False) line.
• testsuite/archive_test.py — update the MockCache mock accordingly.

src/borg/legacy/ is intentionally untouched — the legacy repository/remote keep their real wait/async implementation.

Testing

• pytest src/borg/testsuite/archive_test.py src/borg/testsuite/repository_test.py → all pass.
• Manual smoke: repo-create → create → transfer → check across two local repos, exercising the write_chunk and transfer paths that previously used wait=False. All OK.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.89%. Comparing base (2eef6c8) to head (06b1004).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9782   +/-   ##
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  Coverage   84.88%   84.89%           
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  Files          92       92           
  Lines       15172    15161   -11     
  Branches     2275     2273    -2     
=======================================
- Hits        12879    12871    -8     
+ Misses       1589     1588    -1     
+ Partials      704      702    -2     

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