DefaultLogger on log/slog; remove logrus

[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

• logger.go: keep the exported Logger interface exactly as is (Debugf(format string, args ...interface{}), Errorf(format string, args ...interface{})) so callers are unaffected.
• Reimplement DefaultLogger on *slog.Logger with a text handler to stderr at slog.LevelDebug, adding the attribute src=easymongo. NewDefaultLogger() signature unchanged. Add NewSlogLogger(l *slog.Logger) Logger so callers can plug in their own slog logger.
• Remove github.com/sirupsen/logrus from go.mod (the connect_test.go fixture is handled by the test-suite issue).
• Optional follow-up, not in scope here: align easymongo's Logger with mongotest's structured Logger so the logrus/zap/zerolog adapter modules from mongotest work for both. Note it in the doc comment as a possible future change.

Implementation details

type slogLogger struct{ l *slog.Logger }
func NewSlogLogger(l *slog.Logger) Logger { return &slogLogger{l: l.With("src", "easymongo")} }
func (s *slogLogger) Debugf(format string, args ...interface{}) { s.l.Debug(fmt.Sprintf(format, args...)) }
func (s *slogLogger) Errorf(format string, args ...interface{}) { s.l.Error(fmt.Sprintf(format, args...)) }
func NewDefaultLogger() *DefaultLogger { /* wraps NewSlogLogger(slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelDebug}))) */ }

Keep the exported DefaultLogger type name so existing references compile; it can embed the slog-backed logger.

Also fix the pre-existing bug at connect.go in the debug monitor: conn.log.Errorf("...: %w. Raw reply: %s", err, e.Reply) uses %w with a non-fmt.Errorf function, which prints %!w(...); change to %v.

Tests first (logger_test.go)

• NewSlogLogger with a slog.NewJSONHandler writing to a buffer: Debugf("x %d", 1) produces a record with level=DEBUG, msg="x 1", src=easymongo; Errorf produces level=ERROR.
• NewDefaultLogger() returns a non-nil Logger and does not panic when called.
• ConnectWith(uri).Debug().Logger(l).Connect() followed by a query results in at least one Debugf call on l (integration, via easymongotest).

Acceptance

• grep logrus go.mod returns nothing.

Depends on: #8 (removes the logrus test fixture).