Driver v2: Distinct returns a DistinctResult that must be decoded

[TopherGopher]

Part of the easymongo refactor (see PLAN.md on the refactor branch).

Working context (read this first)

• Repository github.com/tophergopher/easymongo. Start from branch claude/mongotest-easymongo-refactor-wu2vhm; create a branch issue-<number>-<short-slug> from it and open a pull request back into it. PLAN.md on that branch is the source of truth; read it before writing code.
• Go project, single module, all packages at the repo root except the new easymongotest/ subpackage. The final toolchain target is Go 1.27 (set by Toolchain: go 1.27, latest dependencies, tidy, remove replace directives #10); any Go 1.24 or newer works until then. Before pushing: gofmt -l . prints nothing, go vet ./..., go test -race ./....
• The existing test suite (*_test.go at the root, package easymongo_test, every test function calling setup(t) from common_test.go) is the regression net for this refactor. It must pass after your change. It needs a Docker daemon reachable through the normal Docker environment and the mongo:8 image.
• Order matters. The driver package cannot compile half-migrated (the *mongo.Client type flows from Connection to Database to Collection), so the four driver issues Driver v2: migrate connect.go (mongo.Connect, Disconnect, BSONOptions, write concern) #2, Driver v2: options struct literals become builders; drop MaxTime; ArrayFilters and Delete option changes #3, Driver v2: primitive package merged into bson; replace x/bsonx in index.go #4 and Driver v2: Distinct returns a DistinctResult that must be decoded #5 land together in one pull request, worked in that order. Before them, New easymongotest subpackage wrapping mongotest/v2 #7 and Switch the easymongo test suite to easymongotest #8 must land so the test suite no longer depends on the old github.com/tophergopher/mongotest v0.1.0 module (which itself imports easymongo and breaks the moment easymongo's types change). Until mongotest's v2 module is tagged, use a local replace github.com/tophergopher/mongotest/v2 => ../mongotest/v2 in go.mod and say so in the pull request; Toolchain: go 1.27, latest dependencies, tidy, remove replace directives #10 removes it.
• Method is test-driven: write the tests listed under "Tests first", confirm they fail for the expected reason, then implement. Never delete, skip or weaken an existing test to get green; if a test must change because the public API changed (for example primitive.ObjectID to bson.ObjectID), change only the types.
• Driver v2 reference: https://pkg.go.dev/go.mongodb.org/mongo-driver/v2 and the migration guide https://github.com/mongodb/mongo-go-driver/blob/master/docs/migration-2.0.md. Signatures quoted in these issues were checked against v2.9.0.
• Do not add third-party dependencies unless this issue lists them.
• Tracking issue for the whole refactor: Tracking: easymongo refactor (mongo-driver v2, easymongotest helpers, toolchain) #13. The mongotest side is tracked in Tracking: mongotest refactor (stdlib Docker client, driver v1/v2 modules, lighter dependencies) mongotest#48.
• Commit messages: imperative summary line, blank line, body explaining why. Mention this issue number in the pull request description.

Scope

• find_query.go line 344 (mongoColl.Distinct(ctx, field, filter, opts)) returned ([]interface{}, error) in v1. In v2 it returns *mongo.DistinctResult with Decode(v any) error and Err().
• Update find_distinct.go so the easymongo Distinct(...) API keeps its current signature for callers (whatever it returns today, keep returning that by decoding into it). If the current API returns []interface{}, decode into []any; also add a typed variant DistinctInto(field string, result any) error so callers can decode straight into []string etc.
• Remove the commented-out v1 code at line 221.

Implementation details

Verified in v2.9.0:

func (coll *mongo.Collection) Distinct(ctx context.Context, fieldName string, filter any, opts ...options.Lister[options.DistinctOptions]) *mongo.DistinctResult
func (dr *mongo.DistinctResult) Decode(v any) error
func (dr *mongo.DistinctResult) Err() error
func (dr *mongo.DistinctResult) Raw() (bson.RawArray, error)

Implementation shape in find_distinct.go:

res := coll.mongoColl.Distinct(ctx, field, filter, opts)
if err := res.Err(); err != nil { return nil, err }
var out []any
if err := res.Decode(&out); err != nil { return nil, err }
return out, nil

DistinctInto(field string, result any) error calls res.Decode(result) directly; result must be a pointer to a slice, document that.

Tests first (find_distinct_test.go)

• Distinct over name on the Batman archive fixture returns the six names.
• DistinctInto decodes into []string.
• Distinct with a filter that matches nothing returns an empty slice, not an error.
• Distinct honouring Collation still works (case-insensitive collation collapses Joker/joker).

Acceptance

• Existing distinct tests pass; new ones added.

Depends on: #4. Lands in the same pull request as #2, #3 and #4.