#1597 - Migrate DocumentManager to use ThreadSafeBox

[DPrakashh]

Following the discussion in #1597, this PR migrates the documents dictionary in DocumentManager from a DispatchQueue to a ThreadSafeBox.

This approach was chosen to keep the DocumentManager APIs synchronous while still ensuring thread safety.

Changes:
1-Replaced DispatchQueue with ThreadSafeBox<[DocumentURI: Document]>.
2-Updated all call sites (open, close, edit, etc.) to use .withLock.
3-Marked the Document class as @unchecked Sendable to satisfy ThreadSafeBox requirements.
4-Cleaned up imports (removed Dispatch, added SwiftExtensions).

Note:Verified locally with swift test; all relevant tests passed.

[ahoppen]

Nice. That looks easier than I imagined.

[DPrakashh]

I have pushed the refactored threading model as discussed. The implementation now uses a modernised ThreadSafeBox to manage the document registry.

Key Technical Highlights:
1-Locking Strategy: Replaced the serial DispatchQueue in DocumentManager with ThreadSafeBox. This provides lower-latency synchronization for document lookups and edits.
2-Type Safety: Relaxed ThreadSafeBox constraints to support non-Sendable storage while enforcing Sendable on the withLock return value. This allowed me to remove @unchecked Sendable from the Document class.
3-Compiler Satisfaction: Used nonisolated(unsafe) for internal box storage, manually protected by NSLock, to satisfy Swift 6 strict concurrency checks.

Verification: Verified the build and successfully ran "swift test".

[DPrakashh]

Hi @ahoppen,
I wanted to check if it's alright if I contribute from my college-linked account
Link - account starting with Mark 3? I'm using this account specifically to access GitHub Education benefits during my studies, and I'd like to keep my academic work on this profile until I graduate.

I'll ensure that my name and commit identity remain consistent so you can still track my progress from Mark 2 easily. Does this switch work for you? - as my account got approved for education pack of GitHub😊

Thanks!

[ahoppen]

Nice. Looks good to me 👍🏽

[ahoppen]

@swift-ci Please test

[ahoppen]

I wanted to check if it's alright if I contribute from my college-linked account

Absolutely! Contribute with whichever GitHub account you prefer.