chore: add missing unit tests for isNotNull and lineage edge cases in cht-datasource

[shivv23]

Summary

Adds missing unit test coverage for the isNotNull type guard and edge cases for lineage utility functions in the @medic/cht-datasource shared library.

Details

Based on a test coverage audit of the cht-datasource library, the following gaps were identified and filled:

1. isNotNull type guard — 8 test cases

shared-libs/cht-datasource/src/libs/core.ts

Had zero direct unit tests despite being an exported type guard used across the library (filter(isNotNull) appears in multiple lineage functions). Added 8 parameterized cases:

Input	Expected
null	false
undefined	false (type guard passes, no narrowing)
'value'	true
0	true
''	true
false	true
{}	true
[]	true

2. getLineageDocsById edge cases — 3 tests

Had only 1 happy-path test. Added:

• Document not found → empty array returned
• Database query error → error propagated
• Null docs in query results → preserved in response

3. minifyLineage edge cases — 3 tests

Added tests for:

• Deeply nested parent chain (recursive minification verified)
• Extra field stripping (parent.name, type removed, only _id + parent._id kept)
• Doc with no parent field

4. minifyDoc edge cases — 3 tests

Added tests for:

• Extra fields stripped from full doc
• Doc with no parent field
• Original document immutability (minifyDoc must not mutate the input)

All tests follow the existing patterns: chai expectations, sinon stubs, and async/await where needed.

Testing

cd shared-libs/cht-datasource
npm test