fix(test): make external model fixtures hermetic

[Rishabh060105]

Closes #153

Summary

This change makes the test suite hermetic for the remaining external-model paths.

It replaces live model downloads in the affected tests with cached @models... fixtures, following the same naming pattern already used in Concerto tests and existing Accord Project archive fixtures.

Changes

• add cached external Concerto model fixtures under test/models
• add a shared test helper to load vendored external models into a ModelManager
• mock external model fetches for the archive-based test path using cached local fixtures
• update TemplateMarkInterpreter.test.ts to stop calling updateExternalModels() at runtime
• update TemplateArchiveProcessor.test.ts to resolve external model fetches from local cached fixtures

Test

npm test -- --runInBand --coverage=false

Results:

• 11 test suites passed
• 63 tests passed
• 38 snapshots passed

[Rishabh060105]

Hi @mttrbrts, the fix follows the Concerto-style cached external model pattern.

What changed:

• vendored the required external models as cached @models... fixtures under test/models
• replaced the runtime updateExternalModels() dependency in TemplateMarkInterpreter.test.ts with local cached model loading
• added a shared helper for offline external model resolution in tests
• updated TemplateArchiveProcessor.test.ts to resolve external model fetches from cached local fixtures instead of live downloads

I also verified the full suite locally after the change:

npm test -- --runInBand --coverage=false