Make _ThreadLocal.value public under OpenSwiftUI SPI

[Kyle-Ye]

Summary

• Expose _ThreadLocal.value property as public API under the @_spi(OpenSwiftUI) annotation
• This allows the OpenSwiftUI framework to access the thread-local storage value while keeping it hidden from general public API

Changes

• Added @_spi(OpenSwiftUI) and public modifiers to the value property in both conditional compilation blocks:
  • The OPENOBSERVATION_SWIFT_TOOLCHAIN_SUPPORTED version
  • The fallback platform-specific implementation

Test Plan

• Verify that the code compiles successfully
• Confirm that OpenSwiftUI can now access _ThreadLocal.value when importing with @_spi(OpenSwiftUI)
• Ensure the property remains hidden from regular public API consumers