chore: Move the whole package to the Swift 6 language mode

[keelerm84]

Completes the Swift 6 migration started in SDK-2593: the library target was
already on the v6 language mode while the test target stayed on v5 as a
temporary seam until the test doubles were made Sendable. This makes the whole
package build and test under the v6 language mode.

• Rework EventSink from a mutating value type backed by a semaphore + serial
  queue into a final, @unchecked Sendable class guarded by an NSCondition, with
  a reset() so the shared instance can be reused across tests.
• Make MockHandler and RequestHandler final, and EventSink properties immutable.
• Replace MockingProtocol's static mutable requested with a static let that is
  reset() between tests rather than reassigned.
• Add a lock-protected Box so tests can mutate counters/actions captured by
  the @sendable connectionErrorHandler closures.
• Turn DummyError into a struct so it is trivially Sendable.
• Declare the v6 language mode once at the package level via
  swiftLanguageModes: [.v6] instead of per target.

---

Note

Low Risk
Changes are confined to Package.swift and test support code; production library sources are unchanged aside from shared language mode.

Overview
Finishes the Swift 6 migration by applying swiftLanguageModes: [.v6] at the package level instead of per-target settings, so tests compile under the same rules as the library.

Test infrastructure is reworked for v6 concurrency: EventSink becomes a lock-guarded @unchecked Sendable class (replacing a mutating struct with semaphore/queue), with reset() so MockingProtocol.requested can stay a static let. MockHandler is final with an immutable events sink; Box<T> lets testDispatchError mutate counters inside @Sendable connectionErrorHandler closures; DummyError becomes a struct for trivial Sendability.

^{Reviewed by Cursor Bugbot for commit 49eec47. Bugbot is set up for automated code reviews on this repo. Configure here.}