Fix WorkflowReplayer for method-expression local activities

[chiliec]

What

Closes #2589.

WorkflowReplayer could not replay workflows that execute a local activity referenced by a method expression, e.g.

ExecuteLocalActivity(ctx, (*Activities).Foo)

Replaying such a workflow failed with:

expected 2 args for function: Foo but found 0

Root cause

In internal/workflow.go, ExecuteLocalActivity looks the activity up in the registry on replay. The string-name branch (localCtx.fn == nil) already has an IsReplayNamespace(...) fallback that substitutes a dummy function when the activity is unregistered, so replay works. The isMethod branch had no such fallback: on a registry miss it called validateFunctionArgs(localCtx.fn, args, ...) against the unbound method expression, which carries an extra receiver argument, and failed.

WorkflowReplayer only exposes RegisterWorkflow/RegisterDynamicWorkflow (activities can't be registered on it), so a method-expression local activity can never be made to hit the registry — the replay always failed.

Fix

Extend the existing IsReplayNamespace dummy-function fallback to the isMethod branch, mirroring the string-name branch exactly. One branch added in internal/workflow.go (+8 lines).

Tests

Added TestReplayWorkflowHistory_MethodLocalActivity in internal/internal_worker_test.go — a workflow that calls ExecuteLocalActivity(ctx, (*replayLocalActivities).ReplayLocalActivity) replayed through NewWorkflowReplayer, following the existing TestReplayWorkflowHistory_LocalActivity pattern.

Validation (real results, Go 1.25.4)

$ go test ./internal/ -run 'TestInternalWorkerTestSuite/TestReplayWorkflowHistory_MethodLocalActivity' -count=1
ok  	go.temporal.io/sdk/internal	0.059s

Verified genuine RED→GREEN — reverting only the internal/workflow.go fix (keeping the test) reproduces the exact reported error:

$ git stash push internal/workflow.go   # keep the test, drop the fix
$ go test ./internal/ -run '.../TestReplayWorkflowHistory_MethodLocalActivity' -count=1
ERROR activity failed with error. Error expected 2 args for function: ReplayLocalActivity but found 0
--- FAIL: TestInternalWorkerTestSuite/TestReplayWorkflowHistory_MethodLocalActivity
FAIL

Restoring the fix → green again. Full worker suite passes with no regressions:

$ go test ./internal/ -run 'TestInternalWorkerTestSuite' -count=1
ok  	go.temporal.io/sdk/internal	1.104s

go vet ./internal/ clean; both changed .go files are gofmt-clean (I intentionally did not reformat unrelated lines). Added a ### Fixed CHANGELOG.md entry to satisfy the changelog check.

First-time contributor here — happy to adjust the approach (the alternative discussed in the issue, adding RegisterActivity to WorkflowReplayer, is more involved and API-surface-changing; this mirrors the existing string-name fallback with minimal risk).

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