feat: add @Sendable annotations to closure parameters for Swift 6 compliance

[sanchitmehtagit]

Motivation

Swift 6 strict concurrency requires closures that escape across
concurrency boundaries to be @Sendable. Without this, the compiler
emits warnings (Swift 5 mode) or errors (Swift 6 mode) when SDK
callbacks are passed into DispatchQueue.async, Task, or any
other concurrent context — which is exactly how the SDK uses them
internally (e.g. dispatchOnMain, SynchronizationBarrier,
IDTokenValidator).

This brings the callback-based public API into line with Swift 6
strict concurrency, matching the scope of #1095, and ensures users
adopting Swift 6 don't encounter unexpected compiler errors at their
call sites.

Summary

• Add @Sendable to all escaping closure/callback parameters across
  the public API and internal SDK utilities
• Fix the Requestable mocking example in V3_MIGRATION_GUIDE.md
  which was no longer valid against the updated protocol
• Add a new @sendable callback parameters section to the Swift 6
  Concurrency chapter of the migration guide, clarifying that only
  custom protocol implementations (mocks/test doubles) require changes

Files changed

File	Change
Request.swift	Callback typealias + handle property → @Sendable
Requestable.swift	start(_:) callback → @Sendable
WebAuth.swift	onClose, start, logout callbacks → @Sendable
Auth0WebAuth.swift	onCloseCallback property + onClose, start, logout → @Sendable
IDTokenValidator.swift	JWTAsyncValidator protocol + implementations → @Sendable
IDTokenSignatureValidator.swift	validate(_:callback:) → @Sendable
SynchronizationBarrier.swift	pendingOperations + execute(_:) → @Sendable
OAuth2Grant.swift	credentials(from:callback:) → @Sendable
LoginTransaction.swift	FinishTransaction typealias → @Sendable
Shared.swift	dispatchOnMain input + return types → @Sendable
CredentialsManager.swift	All public/private callback parameters → @Sendable
V3_MIGRATION_GUIDE.md	New section + mocking example fix

Test plan

• Build succeeds with SWIFT_STRICT_CONCURRENCY = complete
• Existing tests pass without modification
• No new warnings introduced under Swift 6 mode