fix: UserAgent actor isolation

[stevensJourney]

closes: #109

UIDevice.current.systemVersion is MainActor isolated, which means we should safely use this in other async contexts. This PR cleans up the UserAgent method to cater for potentially asynchronously accessing this property.

Here are some samples for PowerSync service useragent logs

powersync-swift/1.10.0 iOS/26.2
{"client_id":"a8cf0c70-4b84-4e7e-b4d4-445549396f11","duration_ms":26200,"level":"info","message":"STREAM /sync/stream","rid":"s/019c0a5f-adb7-7077-8d49-2bfe2e347cc4","timestamp":"2026-01-29T15:29:54.711Z","user_agent":"powersync-swift/1.10.0 iOS/26.2","user_id":"50ada944-f6af-48dc-9ec5-549bb88aa396"}

powersync-swift/1.10.0 macOS/26.2
{"app_metadata":{},"client_id":"7ed923f6-968e-46a2-8be4-590d5aa5c904","client_params":{},"level":"info","message":"Sync stream started","rid":"s/019c0a61-6495-717f-aab7-37ea557cbb49","timestamp":"2026-01-29T15:31:20.857Z","user_agent":"powersync-swift/1.10.0 macOS/26.2","user_id":"50ada944-f6af-48dc-9ec5-549bb88aa396"}

powersync-swift/1.10.0 watchOS/11.5
{"app_metadata":{},"client_id":"ae638d96-e625-41c1-a5f3-f5697b193f40","close_reason":"client closing stream","data_sent_bytes":674,"data_synced_bytes":1454,"encoding":"permessage-deflate","large_buckets":{},"level":"info","message":"Sync stream complete","operation_counts":{"clear":0,"move":0,"put":2,"remove":0},"operations_synced":2,"rid":"s/019c0a65-ed3d-72dc-bbd2-ef06aee4b07f","stream_ms":10212,"timestamp":"2026-01-29T15:36:28.204Z","user_agent":"powersync-swift/1.10.0 watchOS/11.5","user_id":"50ada944-f6af-48dc-9ec5-549bb88aa396"}

powersync-swift/1.10.0 tvOS/26.2
{"app_metadata":{},"client_id":"36c5460f-ac82-46ca-a0d1-c95be73d97b3","close_reason":"client closing stream","data_sent_bytes":682,"data_synced_bytes":1456,"encoding":"permessage-deflate","large_buckets":{},"level":"info","message":"Sync stream complete","operation_counts":{"clear":0,"move":0,"put":2,"remove":0},"operations_synced":2,"rid":"s/019c4683-0e7c-741b-b18a-441092ee07cc","stream_ms":10339,"timestamp":"2026-02-10T07:45:30.338Z","user_agent":"powersync-swift/1.10.0 tvOS/26.2","user_id":"50ada944-f6af-48dc-9ec5-549bb88aa396"}

[simolus3]

We previously had logic for tvOS, but we don't actually list tvOS as a supported platform in our Package.Swift file. Trying to run the tests for tvOS also results in an error

Huh, we're not even compiling the Kotlin framework for tvOS despite the fact that we have it as a supported platform on our KMP SDK? I'll take a look at that.

Anyway, happy with this fix 👍