fix(#2553) : AsyncSequence.asObservable() runs on background thread

[suojae]

Fixes #2553

Problem Description

The current implementation of AsyncSequence.asObservable() can potentially execute on the main thread, which may cause UI blocking issues. This becomes especially problematic when the AsyncSequence performs time-consuming operations.

Changes

Modified AsyncSequence.asObservable() to use Task.detached, ensuring the sequence always runs on a background thread. This change guarantees that AsyncSequence operations won't block the main thread.

Testing

• Successfully passed all existing tests
• Added and passed test cases that verify execution on background threads
• Included additional test cases covering edge scenarios (with sleep operations, error handling)