Covers every constructor, accessor, MarshalZerologObject, and
DetailsMetadata in the namespace error types.

Covers every constructor, Unwrap, GRPCStatus, MarshalZerologObject,
and accessor method in the graph error types.

Covers every constructor, GRPCStatus, and MarshalZerologObject in
the v1 services error types, plus the defaultIfZero helper.
Exercises full and minimal precondition/filter paths for
PreconditionFailedError and CouldNotTransactionallyDeleteError so
all branches in their GRPCStatus details are hit.

Adds tests for the previously-uncovered caveat, query,
reverse-query, count, and lookup-counter wrappers on the checking
replicated reader, the no-replicas degenerate path, and the
multi-replica round-robin selection via selectReplica.

Exercises the previously-uncovered caveat/namespace/count/lookup
wrappers on strictReadReplicatedReader, the replica-without-strict-
read-mode rejection path in NewStrictReplicatedDatastore, and the
primary-fallback branch in LegacyLookupNamespacesWithNames.

Adds error-path coverage for NewRelationshipIntegrityProxy (empty
and malformed key configs, duplicate key IDs, non-expiring expired
keys, current-key-with-expiration panic), full pass-through
coverage for the trivial datastore methods (MetricsID, UniqueID,
CheckRevision, Close, Features, OfflineFeatures, OptimizedRevision,
ReadyState, RevisionFromString, Statistics, Unwrap), reader
pass-throughs (CountRelationships, caveat/namespace reads,
LookupCounters), ReverseQueryRelationships hash-validation
failure, and BulkLoad both in the happy path and with a
pre-hashed relationship that triggers the bug-assertion panic.

Adds tests for the previously-uncovered public walker wrappers:
WalkCaveat, WalkCaveatWithOptions, WalkBaseRelation,
WalkBaseRelationWithOptions, WalkFlattenedSchema, the value-
receiver WalkOptions.WithStrategy, and WalkOptionsBuilder.MustBuild
on an errored builder. Also covers FlattenSchemaWithOptions with
nil input.

Adds direct unit tests for the two previously-uncovered public
functions in pkg/query: the NewSchemaArrow constructor (asserts it
marks the arrow as a schema arrow and configures left/right and
direction) and DatastoreIterator.ReplaceSubiterators (asserts it
panics since the datastore iterator is a leaf).

The pkg/datalayer package previously had no test files; any coverage
came from downstream consumers. Adds direct tests that wrap an
in-memory datastore and exercise:

- defaultDataLayer pass-throughs (ReadyState, Features, Stats, etc.)
- countingDataLayer query/reverse-query counters on both the
  snapshot reader and the RW transaction
- countingDataLayer pass-throughs and DeleteRelationships
- Unary and Stream counting interceptors, with and without exporters
- NewReadOnlyDataLayer write rejection and reader pass-throughs
- ReadWriteTransaction methods: WriteSchema, LegacySchemaWriter and
  each legacy writer method, BulkLoad, RegisterCounter /
  StoreCounterValue / UnregisterCounter

Extends combined_test.go to exercise the previously-uncovered option
setters (RelationshipChunkCacheConfig, RemoteDispatchTimeout,
CaveatTypeSet, Cache, ConcurrencyLimits, GrpcDialOpts,
RelationshipChunkCache) and the upstream-dispatch branch of
NewDispatcher. Adds error-path tests for invalid secondary
upstream hedging delays (parse failure and non-positive value),
invalid secondary dispatch expressions, and missing upstream CA
certificate files.