[local_auth] Issue 166413 ios stop authentication

[Zubii12]

Add stopAuthentication implementation for ios

Fixes flutter/flutter#166413

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[gemini-code-assist]

Code Review

This pull request implements the stopAuthentication method for the Darwin implementation of the local_auth plugin, updating the Pigeon interface, the Dart platform interface, and the native Swift implementation to support cancelling active authentication sessions. It also includes corresponding unit tests and regenerated Pigeon and mock files. A review comment points out a potential race condition in LocalAuthPlugin.swift where activeContext is cleared unconditionally in an asynchronous callback, which could prematurely clear a newly started authentication session.