Improvements to item versioning UI

[ml-evs]

Headline changes:

• permissions updates now mint a new version with a special action
• timestamps are properly written as timestamps when restoring
• more computed fields are ignored to exclude extraneous version updates
• new tests for this
• updates version history UI to show who made the change, the user agent, and the diff

Closes #2010, closes #1784

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.26%. Comparing base (f885624) to head (ae22d0a).

Files with missing lines	Patch %	Lines
pydatalab/src/pydatalab/routes/v0_1/items.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2011      +/-   ##
==========================================
+ Coverage   80.24%   80.26%   +0.02%     
==========================================
  Files          83       83              
  Lines        7637     7647      +10     
==========================================
+ Hits         6128     6138      +10     
  Misses       1509     1509              

Files with missing lines	Coverage Δ
pydatalab/src/pydatalab/models/versions.py	96.66% <100.00%> (+0.05%)	⬆️
pydatalab/src/pydatalab/versioning.py	91.25% <100.00%> (+0.46%)	⬆️
pydatalab/src/pydatalab/routes/v0_1/items.py	84.77% <83.33%> (+0.11%)	⬆️

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[cypress]

datalab    Run #5341

Run Properties:   Passed #5341  •  d8a9a15330 ℹ️: Merge 1c178f7ff5135485063a2267c73f666bc5ac2d34 into f8856245f1b75666e50beb52fc82...

Project	datalab
Branch Review	ml-evs/versioning-tests
Run status	Passed #5341
Run duration	23m 37s
Commit	d8a9a15330 ℹ️: Merge 1c178f7ff5135485063a2267c73f666bc5ac2d34 into f8856245f1b75666e50beb52fc82...
Committer	Matthew Evans
View all properties for this run ↗︎

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Test results
Failures	0
Flaky	0
Pending	0
Skipped	0
Passing	548
View all changes introduced in this branch ↗︎

[be-smith]

I'm still going through the version history modal logic but it works well on my end. Couple of changes suggested based on what behaviour you want?

[Copilot]

Pull request overview

Improves item version auditing, deduplication, restoration, and history presentation.

Changes:

• Versions permission changes and normalizes restored timestamps.
• Reduces spurious snapshots and expands regression coverage.
• Shows version authors, user agents, actions, and field-level diffs.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
webapp/src/server_fetch_utils.js	Updates version comparison API usage.
webapp/src/components/VersionHistoryModal.vue	Adds audit metadata and diff presentation.
pydatalab/tests/server/test_item_versions.py	Tests snapshot minting and permission changes.
pydatalab/src/pydatalab/versioning.py	Refines snapshot comparison and numbering.
pydatalab/src/pydatalab/routes/v0_1/items.py	Versions permissions and fixes restored timestamps.
pydatalab/src/pydatalab/models/versions.py	Adds the permissions-update action.

Suppressed comments (2)

pydatalab/src/pydatalab/routes/v0_1/items.py:1529

• This storage-type correction is not covered by a raw-database assertion. An API response serializes both a BSON datetime and a string as text, so add a restore test that reads the item from MongoDB and verifies last_modified is a datetime.datetime.

    restored_data["last_modified"] = datetime.datetime.now(tz=datetime.timezone.utc)

webapp/src/components/VersionHistoryModal.vue:343

• A failed comparison is converted to changes = [], causing the preview to state that no content changes were recorded after the error dialog closes. Preserve a distinct comparison-error state and render a retry/error message instead of presenting failure as an empty diff.

      } catch (error) {
        console.error("Failed to load version changes:", error);
        this.changes = [];

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