feat: add subscriptionLimits field to Call type

Add an optional `subscriptionLimits` property to the `Call` class that
exposes the organization's subscription limits (including concurrency
limit information) at the time of the call.

Key changes:
- Add `?SubscriptionLimits $subscriptionLimits` property to `Call`
- Add `subscriptionLimits` as an optional constructor parameter
- Deserialize `subscriptionLimits` from the `subscriptionLimits` JSON property

🌿 Generated with Fern

style: use string interpolation in JsonException messages

Replace string concatenation with double-quoted string interpolation
in error messages thrown by JsonDecoder, JsonDeserializer, and
JsonSerializer. The resulting messages are identical at runtime.

Key changes:
- Replace `"..." . $json` with `"...$json"` in JsonDecoder error messages
- Replace `"..." . $type` with `"...$type"` in JsonDeserializer error message
- Replace `"..." . $unionType` with `"...$unionType"` in JsonSerializer error message

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feat!: remove public classes for voicemail detection, call pagination…

…, and retry middleware

This SDK regeneration removes several public classes that were previously
available to consumers. Callers referencing these types will encounter
fatal errors at runtime or compile-time failures.

Migration guidance:
- `UpdateAssistantDtoVoicemailDetection` has been removed. Review updated
  API types for the replacement voicemail detection union type.
- `AssistantOverridesVoicemailDetection` has been removed. Review updated
  API types for the replacement voicemail detection union type.
- `CallControllerFindAllPaginatedRequest` and
  `CallControllerFindAllPaginatedRequestSortOrder` have been removed.
  Update call-listing code to use the replacement request type.
- `SessionsListRequest` has been removed. Update session-listing code to
  use the replacement request type.
- `RetryMiddleware` (internal core class) has been removed. If you were
  referencing this class directly, update your code accordingly.

Key changes:
- Remove `UpdateAssistantDtoVoicemailDetection` union type class
- Remove `AssistantOverridesVoicemailDetection` union type class
- Remove `CallControllerFindAllPaginatedRequest` request class and its `CallControllerFindAllPaginatedRequestSortOrder` enum
- Remove `SessionsListRequest` request class
- Remove `RetryMiddleware` core HTTP client class

🌿 Generated with Fern

Release 0.0.1