Remove the hardcoded `User-Agent` header from the default headers sent
with every SDK request. This is an internal SDK header change with no
impact on public API surface or consumer code.

Key changes:
- Remove `"User-Agent" => "vapi-server-sdk/0.0.2"` from default request headers

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Add an optional `subscription_limits` field to the `Call` type that surfaces
`SubscriptionLimits` data directly on call objects. This is a backward-compatible
addition — existing code that does not reference this field is unaffected.

Key changes:
- Add optional `subscription_limits` field (mapped from `subscriptionLimits`) to `Vapi::Types::Call`
- Field is typed as `Vapi::Types::SubscriptionLimits` and is nullable: false, optional: true

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…VoiceVoiceIdZero modules

Rename two internal enum modules to better reflect their purpose and
align with updated naming conventions. These modules are part of the
public type system and any code referencing them by name will break.

Key changes:
- Rename `Vapi::Types::CartesiaExperimentalControlsSpeedZero` → `Vapi::Types::CartesiaSpeedControlZero`
- Rename `Vapi::Types::FallbackAzureVoiceVoiceIdZero` → `Vapi::Types::FallbackAzureVoiceIdZero`
- Update `CartesiaSpeedControl` and `FallbackAzureVoiceId` union members to reference the new module names

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Co-authored-by: Dhruva Reddy <dhruva2000@gmail.com>

fix: preserve SDK release notification workflow