This PR implements error type recording on attempt spans for improved
observability in `gax-java`, addressing requirements for better failure
analysis.

#### Key Changes

* **`ErrorTypeUtil` Class**: This new utility class classifies errors
based on a defined priority to populate observability attributes.

### Error Classification Priority

The extraction logic determines the error type based on the following
priority:

1. **`google.rpc.ErrorInfo.reason`**: If the error response from the
service includes `ErrorInfo` details, the reason field (e.g.,
`RATE_LIMIT_EXCEEDED`) is used.
2. **Server Status Code**: If no reason is available, it checks for a
server status code. For HTTP, this is the numeric status code (e.g.,
`403`, `503`). For gRPC, this is the status code name (e.g.,
`PERMISSION_DENIED`, `UNAVAILABLE`).
3. **Client-Side Network/Operational Errors**: If it's a client-side
failure, it maps common exceptions to specific enum representations
(e.g., `CLIENT_TIMEOUT`, `CLIENT_CONNECTION_ERROR`).
4. **Language-specific error type**: Falls back to the class simple name
of the exception (e.g., `NullPointerException`).
5. **Internal Fallback**: Defaults to `INTERNAL` if no other
classification applies.

### Exceptions to be Unwrapped

We investigated standard execution wrappers to ensure accurate error
classification in `ErrorTypeUtil`. We only found one exception so far
that needs unwrapping in this context.

#### `UncheckedExecutionException`
Occurs in
[ServerStreamIterator.java](https://github.com/googleapis/sdk-platform-java/blob/main/gax-java/gax/src/main/java/com/google/api/gax/rpc/ServerStreamIterator.java)
when wrapping checked exceptions observed during stream iteration.

```java
    if (last instanceof Throwable) {
      Throwable throwable = (Throwable) last;
      throw new UncheckedExecutionException(throwable);
    }
```

It is also thrown in
[ApiExceptions.java](https://github.com/googleapis/sdk-platform-java/blob/main/gax-java/gax/src/main/java/com/google/api/gax/rpc/ApiExceptions.java)
during synchronous call translation:

```java
  public static <ResponseT> ResponseT callAndTranslateApiException(ApiFuture<ResponseT> future) {
    try {
      return Futures.getUnchecked(future);
    } catch (UncheckedExecutionException exception) {
      if (exception.getCause() instanceof RuntimeException) {
        // ...
      }
      throw exception;
    }
  }
```