Propagate Swift task cancellation to bridged Kotlin async throws calls

[piercifani]

Summary

Cancelling a Swift task that awaits a bridged Kotlin async throws API never reached the Kotlin coroutine. The generated Swift awaited the callback_ function with a plain withCheckedThrowingContinuation, and the generated Kotlin ran the call in a fire-and-forget Task { … }, so a Kotlin implementation parked in suspendCancellableCoroutine (an OkHttp call, a channel receive) ran to completion or to its own timeout before the Swift await resumed. invokeOnCancellation never fired.

This change makes the two sides cooperate:

• Kotlin (callback_ for throwing async functions and properties) runs the call under its own kotlinx.coroutines.Job via withContext(f_job) and returns that job. Cancelling it before the call starts makes withContext throw, so the callback still fires exactly once.
• Swift awaits inside withTaskCancellationHandler, attaches the returned job through the new SkipBridge.BridgedJob helper (Add BridgedJob so cancelling a Swift task cancels the bridged Kotlin coroutine skip-bridge#120) and cancels it from the handler. A throwable delivered after that cancellation surfaces as CancellationError.
• The JNI signature of the throwing callback_ functions changes from …)V to …)Lkotlinx/coroutines/Job;; both sides are generated together.

Non-throwing async APIs are unchanged: Swift cancellation can only surface through a throwing call, and withCheckedContinuation has no way to report it.

skip.lib.Task.cancel() is deliberately not used for this: it only flips the cooperative flag and runs Swift-style cancellation handlers, it does not cancel the underlying Deferred, so a coroutine suspended in suspendCancellableCoroutine would not observe it.

Testing

• SkipSyntaxTests 867/867: the async throws golden cases in BridgeToSwiftTests and BridgeToKotlinTests updated to the new shape, and BridgeSendabilityTests.testGeneratedContinuationCompilesWithoutWarnings now compiles the whole generated function (with a BridgedJob stub) under -warnings-as-errors.
• End to end, with a skip built from this branch driving code generation and Add BridgedJob so cancelling a Swift task cancels the bridged Kotlin coroutine skip-bridge#120 providing BridgedJob and the new sample test: skip-bridge's Robolectric run (SkipBridgeToSwiftSamplesTestsSupportTests.XCSkipTests) cancels a Swift task awaiting a Kotlin function parked in suspendCancellableCoroutine. With the released 1.9.11 generator that test fails ("The await did not resume within 5 s of the cancellation"); with this branch the await resumes with CancellationError and the coroutine's invokeOnCancellation fires, 70/70 pass. Cancelling before the call starts also resumes.

Depends on skiptools/skip-bridge#120 for BridgedJob.

🤖 Generated with Claude Code

[piercifani]

The android-ci (macos-26-intel, 6.4) failure is the cross-repo dependency, not a regression: skip checkup --native builds the sample app with this generator against the released skip-bridge 0.17.3, which has no BridgedJob yet, so skip-ui's generated UserNotifications_Bridge.swift (two async throws APIs) fails with cannot find 'BridgedJob' in scope. The remaining macOS checkup jobs will fail the same way until skiptools/skip-bridge#120 lands and a skip-bridge release carries it; the ubuntu android-ci jobs and both build-plugin jobs pass.