feat: lenient schema validation; unified exception tree; retry config

[2ynn]

Exception trees unification

This PR unifies exception trees by:

• adding a new Google.GenAI.ApiException : HttpRequestException base exception (root)
• reparenting native ClientError/ServerError are reparented under the root exception
• reparenting the gaos exception tree onto the root exception 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).
}

Retries configuration

Client.cs maps the public HttpOptions.RetryOptions (HttpRetryOptions) onto the
embedded gaos SDK's RetryConfig, so one knob governs both native model calls and
interaction/agent/webhook calls. Fields that don't propagate to gaos fall back to gaos
defaults.

HttpRetryOptions field	native Models.*	gaos interactions/agents/webhooks
Attempts	✓	✓ (→ maxRetries)
InitialDelay	✓	✓
MaxDelay	✓	✓
ExpBase	✓	✓
Jitter	✓	✗ (gaos fixed jitterFactor 0.5)
HttpStatusCodes	✓	✗ (gaos fixed 408/409/429/5XX)

⚠️ HttpRetryOptions.HttpStatusCodes is not respected for interaction/agent/webhook calls. Gaos retryable status codes are hardcoded per-operation in the generated Interactions.cs and are only changeable via the spec x-speakeasy-retries.statusCodes. Code sets diverge only on 408/409 (gaos retries, native does not) and 501 (gaos 5XX vs native's explicit 500/502/503/504).

Lenient schema validation

Additionally, lenient schema validation was enabled (csharp.schemaValidation: lenient), meaning typed models are constructed without raising:

• missing required fields are defaulted
• wrong-typed known fields are skipped
• discriminated-union variants with a known discriminator but invalid payload are still constructed as their typed variant
• error bodies are constructed on a best-effort basis as their typed error class with the strict failure recorded on DeserializationException
• malformed streaming frames do not abort the stream.