add_warning: store exceptions given as args as text, not the exception

[ThomasWaldmann]

What

For a BackupWarning, the args of the warning include the caught BackupError exception. Storing that in the global warnings list (add_warning()) kept the exception - and thus its traceback and the frames (with all their locals) of the code that failed - alive for the rest of the borg run, one per warning. For a failed file in borg extract, that is the extract_item() frame including the chunk data that was being written; on a full disk, every remaining file adds another chunk.

Measured with borg extract of 300 files of 2.5 MB whose writes all fail (ulimit -f, so each write gets EFBIG):

	max RSS
master	876 MiB (300 exception objects and 555 MiB of chunk bytes kept alive via their frames)
this PR	426 MiB (same as a normal extraction of that archive)

How

add_warning() stores exceptions given as args as their message text (str(exc)), which is what formatting the warning message with the exception gives anyway. Everything else (the warning_info list, init_ec_warnings(), get_ec()) is unchanged.

Tests

• regression test at the print_warning_instance() level: the exception wrapped by the BackupWarning must be freed afterwards (checked via weakref, after a gc.collect() for PyPy). Fails on master, passes here.
• unit test for the get_ec() warnings logic (one kind of warning, mixed kinds, error precedence, reset)

Note: 1.4-maint has the same add_warning() in helpers/__init__.py, so the same fix applies there.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.80%. Comparing base (a6c45bc) to head (366db25).
⚠️ Report is 18 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10331      +/-   ##
==========================================
+ Coverage   87.72%   87.80%   +0.07%     
==========================================
  Files         103      103              
  Lines       18816    18828      +12     
  Branches     2904     2906       +2     
==========================================
+ Hits        16507    16531      +24     
+ Misses       1605     1598       -7     
+ Partials      704      699       -5     

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