SDK re-initializes a provider already bound to another domain, causing multi-Init

### Observed behavior

Every `Set*Provider` call runs `Init` on the newly registered provider unconditionally - even when that provider instance is already active on another domain.

OpenFeature spec requirement 1.1.2.2: "Provider instances which are already active (because they have been bound to another domain or otherwise) need not be initialized again."

### Expected Behavior

Binding an already-active provider to another domain skips `Init` and reports readiness from the provider's existing state.

### Steps to reproduce

```go
func TestRequirementMultiBound(t *testing.T) {
inited := atomic.Bool{}
sh := &stateHandlerForTests{
// Semaphore must be invoked
initF: func(e EvaluationContext) error {
if inited.Swap(true) {
return fmt.Errorf("ready initialized")
}
return nil
},
}
provider := struct {
FeatureProvider
StateHandler
}{
FeatureProvider: NoopProvider{},
StateHandler: sh,
}

api := newAPI()
t.Cleanup(func() {
_ = api.Shutdown(context.Background())
})

err := api.SetProviderAndWait(t.Context(), provider)
if err != nil {
t.Errorf("can't set default: %v", err)
}
err = api.SetProviderAndWait(t.Context(), provider, WithDomain("domain"))
if err != nil {
t.Errorf("can't set domain: %v", err)
}
}
```
Result:
```sh
--- FAIL: TestRequirementMultiBound (0.00s)
openfeature_test.go:88: can't set domain: failed to initialize named provider "NoopProvider" for domain "domain": ready initialized
```

Start with SetProviderAndWait and newAPI in openfeature_test.go, then trace how provider binding and initialization are handled for the default and named domains. Run the TestRequirementMultiBound reproduction and add regression coverage showing that an already-active provider is not initialized again and reports its existing readiness state.

Written by the indexing model from the issue text.