Fix race condition in LanguageModelSession when streaming responses with NSLock

[mattt]

Alternative to / extension of #126, but using NSLock instead of creating custom lock type (01be769).

[Copilot]

Pull request overview

This PR fixes race conditions in LanguageModelSession by replacing the previous combination of MainActor dispatching and an actor-based RespondingState with a unified NSLock-based synchronization approach. The implementation introduces a private State struct that encapsulates both the transcript and the responding count, protected by a single NSLock. Properties are exposed as computed properties that properly integrate with Swift's Observation framework using access(keyPath:) for reads and withMutation(keyPath:) for writes.

Changes:

• Replaced MainActor dispatch and actor-based synchronization with NSLock for protecting mutable state
• Converted isResponding and transcript from stored to computed properties backed by a locked State struct
• Added comprehensive observation tests to verify that property changes correctly trigger observation notifications

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Sources/AnyLanguageModel/LanguageModelSession.swift	Implemented NSLock-based synchronization, converted properties to computed properties with Observation framework integration, replaced RespondingState actor with State struct
Tests/AnyLanguageModelTests/ObservationTests.swift	Added comprehensive tests for observation behavior including transcript and isResponding tracking, with ThreadSafeValue helper for thread-safe test assertions

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[mattt]

Closing in favor of #126. See #126 (comment)