fix: make ArtifactPlan.videoRecordingEnabled optional

[skylarkdevs]

Problem

Calling vapi.start(assistantId:) against the current production Vapi server fails immediately with .callDidEnd (no transcript, no audio) because the SDK's strict Decodable decoder throws keyNotFound("videoRecordingEnabled") while parsing WebCallResponse.

The server response no longer always nests videoRecordingEnabled under artifactPlan — it has moved to transport.videoRecordingEnabled for many calls — so the key is genuinely absent from the JSON the SDK receives.

Reproduction:

1. Create an assistant in the Vapi dashboard with default settings (no explicit artifactPlan.videoRecordingEnabled)
2. From iOS, call vapi.start(assistantId: "<id>")
3. Observe vapi.eventPublisher emits .callDidEnd(error: decodingError(keyNotFound("videoRecordingEnabled", ...))) ~0.3s after start

Fix

Make ArtifactPlan.videoRecordingEnabled an optional Bool?. The only existing reader is in Vapi.swift:

let isVideoRecordingEnabled = response.artifactPlan?.videoRecordingEnabled ?? false

Swift's optional-chain flattening means the expression's type is unchanged (Bool?), so this line keeps compiling with no callsite update needed.

Impact

• Restores vapi.start() for SDK consumers whose backend doesn't echo artifactPlan.videoRecordingEnabled on every web-call response.
• No behavior change when the key IS present — same value flows through.
• No public API change other than the property becoming optional, which is source-compatible with the existing ?? false pattern.

Test plan

• Builds cleanly against an iOS 18 simulator target
• vapi.start() against a production assistant now produces .callDidStart + transcript instead of .callDidEnd(decodingError)