test: exercise the extension points with a mock external backend

[ehennestad]

The point of the serialization, deserialization and resolution architecture in this stack is that an external library can integrate a graph database or a new format without changing openMINDS_MATLAB. Until now nothing verified that end to end.

This adds a mock external backend written purely against the extension surface promoted two PRs down:

• MockGraphSerializer — a non-JSON-LD record format on openminds.abstract.BaseSerializer; implements only the constructor and formatOutput, inheriting traversal, reference and embedding handling.
• MockGraphDeserializer — implements only parseToStructs on openminds.abstract.BaseDeserializer, inheriting type dispatch and cross-record link wiring.
• MockGraphMetadataStore — openminds.interface.MetadataStore backed by an in-memory record database.
• MockGraphResolver — openminds.interface.LinkResolver holding its database client as instance state, which is what the instance-method resolver contract exists for.

ExternalIntegrationTest drives the five things an external backend has to be able to do:

1. save a collection as one record per node through the custom serializer;
2. load the records back into a single connected graph — values, wired links, controlled terms and embedded values intact;
3. reproduce the canonical JSON-LD document byte for byte after the round trip, which leans on the deterministic node ordering at the bottom of this stack;
4. resolve a reference whose type only the database knows, by replacement — the case that was silently broken before the traversal rework;
5. populate a typed reference in place.

If a change breaks these tests, it breaks every external integration, openminds-kg-sync included. The mock classes double as a worked example of the contract — roughly 200 lines for a complete backend.

The verification run that produced this test also found the Collection.save store bug fixed in #115.

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[github-actions]

Test Results (R2022a)

767 tests   766 ✅  2m 5s ⏱️
 21 suites    1 💤
  1 files      0 ❌

Results for commit c43a2c8.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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##             add-resolver-reregistration     #118   +/-   ##
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  Coverage                               ?   80.02%           
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  Files                                  ?      422           
  Lines                                  ?     4066           
  Branches                               ?        0           
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  Hits                                   ?     3254           
  Misses                                 ?      812           
  Partials                               ?        0           

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[github-actions]

Test Results (R2026a)

767 tests  +767   766 ✅ +766   1m 57s ⏱️ + 1m 57s
 21 suites + 21     1 💤 +  1 
  1 files   +  1     0 ❌ ±  0 

Results for commit c43a2c8. ± Comparison against base commit bbf211b.

♻️ This comment has been updated with latest results.