repository: clear error for borg 1.x repositories, fixes crash w/o --from-borg1

[ThomasWaldmann]

Description

Opening a borg 1.x repository with borg 2 without --from-borg1 crashed with a "Local Exception" traceback. Examples are borg -r /path/to/v1repo repo-info, or borg transfer --other-repo v1repo without --from-borg1. A borg 1.x repository has a config file where borg 2 expects the config/ namespace, so loading the config/config store object failed differently per platform and backend:

• local repository: NotADirectoryError: [Errno 20] Not a directory: '.../config/config'
• ssh://: the remote borg serve --rest answered HTTP 500 -> requests.exceptions.HTTPError
• on Windows, both came out as "not found" instead (ObjectNotFound / HTTP 404), which borg reported as InvalidRepository

borgstore 0.6.4 (borgbackup/borgstore#228) reports a name below an object as ObjectNotFound on all platforms. So this PR requires borgstore ~= 0.6.4, and a borg 1.x repository now always ends up in the StoreObjectNotFound branch of Repository._load_config. There, it looks at config via the store backend (Store.info only takes names below a namespace). If config is a file, it raises the new Repository.LegacyRepository error (rc 29):

... looks like a borg 1.x repository, use --from-borg1 to access it (e.g. with borg transfer).

Otherwise it continues as before (a store without a repository config). If the probe itself fails with a backend or OS error, it's treated as "not a borg 1.x repository".

A remote borg serve that still uses an older borgstore answers such a request with HTTP 500. In that case, the user still gets the traceback.

Same symptom as the closed #9010 and #8652 (which involved older borg 2 betas).

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate: test_open_legacy_repository[file|ssh] in repository_test.py; LegacyRepository added to the error list in docs/internals/frontends.rst; borgstore requirement bumped in pyproject.toml and docs/installation.rst
• Tests pass (run tox or the relevant test subset): the test suite (except mount/FUSE) passes locally on macOS with borgstore 0.6.4
• Commit messages are clean and reference related issues

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.38%. Comparing base (db0c4ac) to head (4164c09).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/repository.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10418   +/-   ##
=======================================
  Coverage   88.38%   88.38%           
=======================================
  Files         103      103           
  Lines       19029    19039   +10     
  Branches     2961     2962    +1     
=======================================
+ Hits        16819    16828    +9     
- Misses       1538     1540    +2     
+ Partials      672      671    -1     

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