fix(mock): exclude null field values from != queries like Firestore

[detail-app]

Detail bug report: View on Detail

Closes #99

Bug

The mock's != query operator in matchesWhere included documents whose field value was explicitly null, whereas real Firestore excludes them. The branch only guarded against missing fields (value !== undefined), so where(field, '!=', V) returned a strict superset of production whenever a field was null. This is a real, reachable shape: Firestore.Optional/Firestore.OptionalNull encode Option.none() as a literal null field on write, so any != query over such a field diverged between the mock and Firestore. A test asserting on the result set (a length, or "all results have a present status") could pass against the mock and fail in production (or vice-versa).

Fix

packages/mock/src/lib/firestore/query-filter.ts — the != branch now excludes both null and undefined field values, matching Firestore's documented semantics ("null field values do not match != clauses, because x != null evaluates to undefined"):

case '!=':
  if (value === null || value === undefined) return false;
  return !equals(value, where.value);

The adjacent == branch is unchanged — it already handles null correctly (equals(null, nonNull) is false, equals(null, null) is true). Also updated packages/mock/README.md to drop != from the list of "simplified" null semantics (only not-in remains listed; its analogous null gap is out of scope for this change).

Note: the bug report suggested a where.value === null guard so != null matches no documents. Emulator cross-validation disproved this — real Firestore returns the present, non-null-field documents for != null, not the empty set — so that guard is intentionally omitted (see Testing).

Testing

• Unit tests (query-filter.spec.ts): added a fixture mixing present, explicit-null, and missing status values; the bug case where('status','!=','active') now returns only the non-null non-matching doc, plus regression guards for == null (includes explicit-null) and == 'active' (excludes null/missing). The existing all-non-null != spec still returns ['1','4'] (no regression).
• E2e (layer.spec.ts): through Firestore.makeRepository with an Firestore.Optional(Schema.String) field, where('status','!=','active') returns ['B'] (not ['B','C']); the Option.none() doc round-trips as Option.none(), proving the store holds an explicit null field.
• Bug-catching check: reverting the one-line fix makes the new unit test fail with Expected ['b'] Received ['b','c']; restoring it passes.
• Full mock suite, typecheck, lint, and build all pass.
• End-to-end against the real backend: ran the scenarios against the Firebase Firestore emulator via a temporary Admin-layer spec (not committed). where('status','!=','active') returned ['B'] on Firestore, matching the mock. It also confirmed where('status','!=',null) returns the non-null-field documents ['A','B'] (not []), which is why the where.value === null guard was not added.

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the previous documentation concern is resolved and no actionable new defects remain.

Summary

This PR aligns the mock Firestore != operator with Firestore by excluding explicit null and missing field values.

• Updates the query predicate while preserving != null behavior for present, non-null values.
• Adds focused predicate and repository-level regression coverage.
• Updates the mock limitations documentation.
• Corrects the prior misleading emulator-test reference by describing the validation as manual.

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