EventDetails.fromProviderEventDetails drops errorCode, so API-level handlers never see it

### Summary

`EventDetails.fromProviderEventDetails(...)` does not copy `errorCode`, so a handler
registered at API level — `OpenFeatureAPI.onProviderError(Consumer)` —
always sees `details.getErrorCode() == null`, even when the provider explicitly emitted
one.

`message`, `flagsChanged`, `providerName` and `eventMetadata` all arrive intact. Only
`errorCode` is lost, and it is lost silently: `EventDetails` extends
`ProviderEventDetails`, so `getErrorCode()` compiles and returns `null` rather than
failing to compile.

### Environment

- `dev.openfeature:sdk` **1.22.0**

### Steps to reproduce

Emit an error event carrying an explicit code from any `EventProvider` (here a probe
extending `InMemoryProvider`, which is already an `EventProvider`):

```java
probe.emitProviderError(ProviderEventDetails.builder()
.errorCode(ErrorCode.PROVIDER_NOT_READY)
.message("connect refused")
.build());
```

Log it from an API-level handler:

```java
OpenFeatureAPI.getInstance().onProviderError(details ->
log.error("event=PROVIDER_ERROR provider={} message={} error_code={}",
details.getProviderName(), details.getMessage(), details.getErrorCode()));
```

Observed:

```
event=PROVIDER_ERROR provider=InMemoryProvider message=connect refused error_code=null
```

`message` arrives, which rules out "the event never got delivered".

### Root cause

`EventDetails.fromProviderEventDetails(...)` is the only path from a provider-emitted
`ProviderEventDetails` to the `EventDetails` handed to API-level handlers, and its
builder chain simply does not mention `errorCode` (`EventDetails.java` on `main`):

```java
static EventDetails fromProviderEventDetails(
ProviderEventDetails providerEventDetails, String providerName, String domain) {
return builder()
.domain(domain)
.providerName(providerName)
.flagsChanged(providerEventDetails.getFlagsChanged())
.eventMetadata(providerEventDetails.getEventMetadata())
.message(providerEventDetails.getMessage())
.build();
}
```

`ProviderEventDetails` has four fields; three of them are copied. Decompiling 1.22.0
shows the same five-field builder chain, so the observed behaviour and the source agree,
and the two lines of evidence are independent of each other.

### Why it cannot be worked around

- API-level handlers receive `EventDetails`; the original `ProviderEventDetails` is not
reachable from there.
- There is no public way to observe a provider's events directly —
`EventProvider.setEventProviderListener` and `EventProvider.attach` are both
package-private.
- Recovering the code by parsing `message` is not viable: that text is entirely up to
each provider.

So until this is fixed, an application consuming provider events in Java has no error
code available at all.

### Note: the Go SDK does not drop it

`openfeature.EventDetails` in the Go SDK carries `ErrorCode` directly, so the
equivalent handler there does receive it. This is an SDK-level divergence between the
two implementations rather than a difference in how applications are written.

### Suggested fix

Add the missing line to the builder chain:

```java
.errorCode(providerEventDetails.getErrorCode())
```

One line, and I checked the two things that would have made it bigger than that.

**`errorCode` is the only field affected.** `ProviderEventDetails` declares exactly four
fields — `flagsChanged`, `message`, `eventMetadata`, `errorCode` — and the builder chain
transfers the first three. There is no second omission, so this is a missing line rather
than a conversion that needs realigning.

**Populating it does not make SDK-generated events ambiguous.** The events the SDK raises
itself build a `ProviderEventDetails` carrying no error code
(`OpenFeatureAPI.java:307` and `:320`), so their `getErrorCode()` stays `null` exactly as
it is today. The only thing that changes is that a code a provider explicitly set now
survives the conversion.

### Still present on `main`

Confirmed by reading the source at `5bf9f56`, not only the 1.22.0 bytecode:
`EventDetails.fromProviderEventDetails` still has no `.errorCode(...)` in its builder
chain, and all three call sites (`OpenFeatureAPI.java:531`, `:535`, `:543`) go through it.

Happy to open a PR.

Start in EventDetails.java at fromProviderEventDetails and check the three call sites in OpenFeatureAPI.java. Reproduce the provider error with InMemoryProvider and verify that an explicit ErrorCode reaches the API-level handler; done means the code is preserved while events without one still expose null.

索引モデルが issue の本文から書いたものです。