Report malformed tree entries as object corruption

[betacatsling]

Tree.load() classifies JSON decoding failures as corrupt objects but lets malformed list entries escape as KeyError, AttributeError, TypeError or ValueError. Callers such as _try_load() then stop instead of trying another database.

Translate those entry-deserialization errors into ObjectFormatError, preserving the cause. Tests cover malformed entry structures and a real two-database fallback from a corrupt manifest to a valid copy. All five regressions fail on the unchanged base.

Validation on Linux/Python 3.12: 196 tests passed, 32 skipped; Ruff format/lint, project mypy and diff checks passed. No remote storage calls were used. AI assistance was used for implementation and verification.

Prerequisite and revalidation: this branch now includes the shared CLI compatibility fix in #672 (4551489) followed by the original focused patch. This lets a fresh [tests,cli] installation run without external Click. The common change is separately reviewable and will disappear from this PR's diff once #672 is merged.

Revalidated the combined branch on Python 3.12 / Typer 0.27.2 with external Click absent: 202 passed / 32 skipped, mypy passed, and full source/test lint plus formatting passed with the repository-pinned Ruff 0.15.2. The benchmark workflow's base-branch step still needs #672 merged to main; it runs before the PR's code.

Fixes #669.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.33%. Comparing base (e38c8c2) to head (27a678e).
⚠️ Report is 72 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #670      +/-   ##
==========================================
+ Coverage   72.42%   74.33%   +1.90%     
==========================================
  Files          67       66       -1     
  Lines        5132     5431     +299     
  Branches      605      624      +19     
==========================================
+ Hits         3717     4037     +320     
+ Misses       1227     1198      -29     
- Partials      188      196       +8     

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