This update adds conversation type support to TwilioSmsChatTransport and introduces a dial timeout option for TransferPlan. These enhancements provide better control over chat conversation types and transfer call behavior.

Key changes:
- Add TwilioSmsChatTransportConversationType enum with "chat" literal type
- Add conversation_type field to TwilioSmsChatTransport to specify call type
- Add dial_timeout field to TransferPlan for SIP DIAL operation timeout control
- Update User-Agent header to use version placeholder instead of hardcoded value
- Export new conversation type in module __all__ lists

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Unable to analyze changes with AI, incrementing PATCH version.

The SDK now supports aiohttp as an alternative async HTTP transport backend.
When the `aiohttp` optional extra is installed (`pip install vapi_server_sdk[aiohttp]`),
the `AsyncVapi` client will automatically use `httpx-aiohttp` as its underlying
transport. Users who prefer the standard httpx client are unaffected — behaviour
is unchanged unless the extra is explicitly installed.

Key changes:
- Add new `aiohttp` optional extra (`aiohttp>=3.10,<4` + `httpx-aiohttp==0.1.8`)
- Introduce `DefaultAioHttpClient` and `DefaultAsyncHttpxClient` convenience classes (exported from top-level `vapi` package)
- Add `_make_default_async_client()` helper that auto-detects and prefers `httpx-aiohttp` when available
- Add pytest marker and conftest fixture to skip aiohttp tests when the extra is not installed

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Add an optional `subscription_limits` field to the `Call` model that
exposes org-level subscription limits (including concurrency limit
information) at the time of a call. This surfaces a new `SubscriptionLimits`
type and wires it as an aliased optional field on `Call`.

Key changes:
- Add `SubscriptionLimits` import to `call.py`
- Add optional `subscription_limits` field (aliased as `subscriptionLimits`) to the `Call` model
- Field defaults to `None` and includes description of concurrency limit context

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Remove explicitly set `"content-type": "application/json"` headers from
multiple raw client request calls across the SDK. These headers are
already handled by the underlying HTTP client when a JSON body is
present, making the explicit declarations redundant.

Key changes:
- Remove hardcoded `content-type: application/json` headers from `RawAssistantsClient` and `AsyncRawAssistantsClient`
- Remove same redundant headers from `RawEvalClient`, `RawInsightClient`, `RawObservabilityScorecardClient`, `RawPhoneNumbersClient`, `RawSquadsClient`, `RawStructuredOutputsClient`, and `RawToolsClient`
- Applies to both sync and async variants of all affected clients

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