[Speech Engine] Add full response to Speech Engine API calls

[PaulAsjes]

We currently override the response from the get, create and update methods to return a Speech Engine resource instead of a response. That also means that the data returned from the API is lost to the end user. This adds it back as part of a config object that can be accessed. e.g.

engine = await elevenlabs_client.speech_engine.create(
    name="SDK test speech engine",
    speech_engine={
        "ws_url": "wss://pma.ngrok.dev/ws",
    },
    overrides={
        "first_message": True,
    },
)
});

print("Engine created with overrides:", engine.config.overrides)

await engine.serve(...);

Also includes missing overrides hash in the API request models.

---

Note

Low Risk
Low risk: additive changes to the Speech Engine client surface (new optional overrides parameter and a stored response object) plus a patch version bump, with minimal impact on existing request/response handling.

Overview
Speech Engine resources returned from create/get/update now retain the full SpeechEngineResponse: SpeechEngineResource accepts the API response and exposes it via engine.config for downstream access (e.g., engine.config.asr, engine.config.overrides).

Adds an optional overrides parameter to Speech Engine create (sync/async) and wires it through to the raw POST body as overrides, using the new SpeechEngineConversationInitiationClientDataConfig model. Also bumps SDK version to 2.49.1 and updates the User-Agent/X-Fern-SDK-Version headers accordingly.

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