feat: Implement attended transfer functionality in SIP UA

[COACHIKO]

• Added attended transfer events to manage transfer states (trying, progress, accepted, failed).
• Introduced Replaces class to handle SIP Replaces header for attended transfers.
• Created AttendedTransferScreen to manage the attended transfer UI flow.
• Updated SIP UA helper to support attended transfer methods.
• Enhanced sip_ua.dart exports to include new event and replaces classes.

[Copilot]

Pull request overview

This PR implements attended transfer functionality (RFC 5589) for the SIP UA library, allowing users to consult with a third party before completing a call transfer. The implementation adds comprehensive SIP signaling support and a complete UI flow for managing attended transfers.

Key Changes:

• Added SIP Replaces header support for attended transfers with proper dialog identification
• Implemented attended transfer methods in the Call helper with callbacks for transfer lifecycle events
• Created a dedicated UI screen to manage the consultation call and transfer completion flow

Reviewed changes

Copilot reviewed 8 out of 10 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
lib/src/sip_ua_helper.dart	Added attendedTransfer() and attendedTransferWithParams() methods with dialog info getters
lib/src/replaces.dart	New class representing SIP Replaces header for dialog replacement
lib/src/event_manager/attended_transfer_events.dart	New event classes for attended transfer state tracking
lib/sip_ua.dart	Exported new attended transfer events, refer events, replaces class, and refer subscriber
example/lib/src/attended_transfer_screen.dart	New screen managing the attended transfer UI flow with consultation call handling
example/lib/src/callscreen.dart	Enhanced with attended/blind transfer dialog options and platform-specific speaker handling
example/lib/src/dialpad.dart	Added logic to prevent automatic navigation during attended transfer
example/macos/*	CocoaPods configuration updates (generated files)

Files not reviewed (1)

• example/macos/Runner.xcworkspace/contents.xcworkspacedata: Language not supported

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[Copilot]

The toEncodedString() method in the Replaces class duplicates the encoding logic that already exists in refer_subscriber.dart (lines 34-37). This violates the DRY principle. Since refer_subscriber.dart is the code that actually uses this encoding, consider whether this method is needed or if the logic should be consolidated in one place.

Suggested change

	/// Builds the encoded Replaces parameter for a Refer-To URI.
	///
	/// Returns a properly formatted and URI-encoded string that can be
	/// appended to a Refer-To header.
	///
	/// Format: `Call-ID;to-tag=xxx;from-tag=yyy` (URI encoded)
	String toEncodedString() {
	String replaces = callId;
	replaces += ';to-tag=$toTag';
	replaces += ';from-tag=$fromTag';
	return Uri.encodeComponent(replaces);
	}

[COACHIKO]

@copilot open a new pull request to apply changes based on the comments in this thread