stream-chat-swift Swift 6: complete concurrency checking (LLC and UIKit)

[!IMPORTANT] StreamChatUI changes are in #3660 and will be merged to here. This PR can be reviewed already.

🔗 Issue Links

Resolves IOS-735

🎯 Goal

Set Swift version to 6 and implement complete concurrency checking

📝 Summary

Sendable conformance to types
@Sendable to completion handlers
Many thread-safety related changes to @unchecked Sendable types (internal classes have manual handling and found many which did not guard that state properly)
Fixes warnings in the demo app (adding @Sendable to completion handlers creates warnings at callsites)
Backported AllocatedUnfairLock which is needed for static concurrency safe properties (our Atomic property wrapper can't be used)

🛠 Implementation

🧪 Manual Testing Notes

Manual regression testing round when UIKit changes are merged into this branch.

☑️ Contributor Checklist

[x] I have signed the Stream CLA (required)
[x] This change should be manually QAed
[ ] Changelog is updated with client-facing changes
[ ] Changelog is updated with new localization keys
[x] New code is covered by unit tests
[ ] Documentation has been updated in the docs-content repo

May 05 '25 12:05 laevandus

1 Warning

:warning: Big PR

1 Message

:book: There seems to be app changes but CHANGELOG wasn't modified.
Please include an entry if the PR includes user-facing changes.
You can find it at CHANGELOG.md.

	1 Warning
:warning:	Big PR

	1 Message
:book:	There seems to be app changes but CHANGELOG wasn't modified. Please include an entry if the PR includes user-facing changes. You can find it at CHANGELOG.md.

Generated by :no_entry_sign: Danger

May 05 '25 12:05 github-actions[bot]

SDK Size

`title`	`develop`	`branch`	`diff`	`status`
StreamChat	7.2 MB	7.28 MB	+81 KB	🟢
StreamChatUI	4.72 MB	4.73 MB	+17 KB	🟢

May 05 '25 12:05 Stream-SDK-Bot

SDK Performance

`target`	`metric`	`benchmark`	`branch`	`performance`	`status`
MessageList	Hitches total duration	10 ms	10.85 ms	-8.5% 🔽	🟡
	Duration	2.6 s	2.54 s	2.31% 🔼	🟢
	Hitch time ratio	4 ms per s	4.28 ms per s	-7.0% 🔽	🟡
	Frame rate	75 fps	78.23 fps	4.31% 🔼	🟢
	Number of hitches	1	1.0	0.0% 🟰	🟢

May 05 '25 12:05 Stream-SDK-Bot

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