Clean up onCanSendFeedbackChange

[renal128]

Summary

Per-message feedback can be sent at any time as long as the conversation is connected. But historically it was implemented via some complex logic on the client, so that feedback was only allowed on the last message if that message doesn't have feedback yet - that's purely a client-side limitation that we don't need.

Changes:

• remove onCanSendFeedbackChange and its event-tracking state
• make connected conversation state the sole feedback availability signal
• update feedback documentation and coverage for connected/disconnected behavior

Test plan

• swift test

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Note

Medium Risk
Public API removal breaks apps that relied on onCanSendFeedbackChange; behavior change may allow feedback on older messages while connected, which is intentional but affects UI assumptions.

Overview
Removes client-side feedback gating so apps no longer get onCanSendFeedbackChange or internal tracking (lastAgentEventId, lastFeedbackSubmittedEventId). Feedback is allowed for any eventId while the session is connected; sendFeedback still throws when not connected.

Breaking: ConversationCallbacks.onCanSendFeedbackChange is removed from the public API. UI should enable thumbs up/down based on client.state.isConnected (or equivalent) instead of SDK-driven toggles on agent response, interruption, disconnect, and after submit.

Docs and tests are updated: Usage.md shows ConversationClient + guard client.state.isConnected, and unit tests assert published feedback payloads instead of callback-driven availability.

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