testsuite: decouple legacyrepository_test from RepoObj, refs #8572

[mr-raj12]

Description

legacyrepository_test was importing RepoObj (borg 2) to build test blobs in fchunk/pchunk. That coupling was wrong: borg 1.x blobs have a 1-byte compressor type header followed by the payload, not the Struct("<II") meta/data header from borg 2.

Remove the RepoObj import. fchunk now prepends a 1-byte CNONE type header (0x00) to simulate the generic borg 1.x blob format without real compression. pchunk strips that byte. test_max_data_size updated to account for the 1-byte overhead.

This way, changes to the borg 2 blob format (e.g. OBJ_MAGIC in #9684) no longer touch the legacy test helpers.

refs #8572

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.88%. Comparing base (7f43779) to head (a0494b2).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9687   +/-   ##
=======================================
  Coverage   83.87%   83.88%           
=======================================
  Files          93       93           
  Lines       15586    15586           
  Branches     2335     2335           
=======================================
+ Hits        13073    13074    +1     
  Misses       1785     1785           
+ Partials      728      727    -1     

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[ThomasWaldmann]

The idea is:

• fchunk(data, meta) -> chunk_content -> repo put
• repo get -> pchunk -> data, meta

That fchunk/pchunk is just a simplified way to do something close to what borg usually does, but without having to set up compression and encryption.

[ThomasWaldmann]

LGTM.