fix(compose): correct off-by-one in mergeStream error message

[Dennisadira]

Summary

Fixes #974.

In compose/values_merge.go, the loop asserts vs[i+1] as a streamReader but on failure logs reflect.TypeOf(vs[i]) — the element before the one that failed:

sri, ok_ := vs[i+1].(streamReader)
if !ok_ {
    return nil, fmt.Errorf("(mergeStream) unexpected type. "+
        "expect: %v, got: %v", t0, reflect.TypeOf(vs[i]))  // wrong index
}

Changed vs[i] → vs[i+1] so the error message reports the type of the element that actually failed the assertion.

Test plan

• go test ./compose/... passes

Assisted-by: Claude Sonnet 4.6 noreply@anthropic.com

[CLAassistant]

All committers have signed the CLA.