feat: reformat; unify exception trees; disable gaos retries

[2ynn]

This PR unifies exception trees by:

• adding a new Google.GenAI.ApiException : HttpRequestException base exception
• reparenting native ClientError/ServerError are reparented under the new root
• repaenting the gaos/ tree is onto the same root via injected carrier classes.

System.Exception
└── System.Net.Http.HttpRequestException
    └── Google.GenAI.ApiException                      ← NEW base — single catch
        ├── Google.GenAI.ClientError       (4xx, native)
        │   └── GaosClientError             (injected carrier)
        │       ├── CreateInteractionClientError
        │       ├── GetInteractionByIdClientError
        │       ├── CancelInteractionByIdClientError
        │       └── DeleteInteractionClientError
        ├── Google.GenAI.ServerError        (5xx, native)
        │   └── GaosServerError             (injected carrier)
        │       ├── CreateInteractionServerError
        │       ├── GetInteractionByIdServerError
        │       ├── CancelInteractionByIdServerError
        │       └── DeleteInteractionServerError
        └── Google.GenAI.Gaos.Models.Errors.GaosBaseException   (gaos root, reparented onto ApiException)
            ├── GaosApiException            (gaos default fallback)
            └── ResponseValidationException

As a result a single catch (Google.GenAI.ApiException) now covers every API error:

try {
    await client.Models.GenerateContentAsync(...);
    await client.Interactions.CreateAsync(...);
} catch (Google.GenAI.ApiException e) {
    int code = e.StatusCode;                            // native accessor, both surfaces
    // e is ClientError (4xx) or ServerError (5xx); gaos body still available
    // on the per-op error / carrier (e.g. ((GaosClientError)e).Body).
}

Additionally:

• gaosBuilder.retryConfig(RetryConfig.noRetries()); was set to disable retries on the goas SDK so they don't interfere with the native ScheduleExecutorService
• the generated SDK was reformatted (enableFormatting: true)