feat: Swift 6 strict concurrency compliance for WebAuth Part 2

[sanchitmehtagit]

Continues from #1123. Completes Swift 6 strict concurrency compliance for the WebAuth layer.

handles this : #1134

Changes

Auth0WebAuth — final class → struct

• Converted to struct to satisfy Sendable conformance without @unchecked Sendable
• All builder methods now follow copy-and-return semantics (var copy = self)
• lazy var redirectURL replaced with a computed property backed by _redirectURL — lazy var is not Sendable-safe on structs and would cache a stale URL if useHTTPS() was called after first access
• nonce removed as a mutable stored property; now computed from parameters (or generated randomly if absent), consistent with how state is handled
• barrier is now let (immutable, safe to share across concurrency domains)
• onCloseCallback type updated to @Sendable @MainActor () -> Void
• presentationWindow property added for multi-window support

WebAuth protocol

• Now inherits from Sendable
• Async start() and logout(federated:) marked @MainActor
• onClose(_:) callback parameter is now @Sendable

ASProvider

• asProvider(...) factory marked @MainActor; accepts presentationWindow parameter
• completionHandler wrapped in Task { @MainActor in } for safe actor hop
• ASUserAgent converted to final class with Sendable conformance
• currentSession and presentationWindow marked @MainActor
• start() and finish(with:) dispatch via Task { @MainActor in }

Barrier / LoginTransaction

• Barrier protocol now inherits from Sendable; QueueBarrier conforms via @unchecked Sendable
• FinishTransaction typealias updated to @MainActor @Sendable

Combine start() publisher

• Removed redundant Task { @MainActor in } wrapper — the callback-based start(_:) already dispatches to @MainActor internally; now consistent with how the logout() publisher is implemented

Migration guide (V3_MIGRATION_GUIDE.md)

• Added "WebAuth is now Sendable" section covering the Sendable requirement for custom WebAuth conformances and the value semantics change — simplified around two clear cases: chaining (no change needed) vs imperative calls (must reassign return value)
• Added "WebAuthProvider — now @Sendable @MainActor" to the typealiases section
• Added "@mainactor on async Web Auth methods" section

Test plan

• Build with -strict-concurrency=complete / Swift 6 mode — no concurrency warnings
• Web Auth login and logout flows function correctly on iOS and macOS
• Multi-window apps: presentationWindow(_:) presents the browser in the correct window
• Ephemeral session flag is correctly propagated through the copy-based builder chain
• Custom WebAuth mock/test double compiles after adding Sendable and @MainActor
• Existing unit and integration tests pass

[sanchitmehtagit]

Update Examples.md

[sanchitmehtagit]

Auth0.swift/EXAMPLES.md

Line 216 in 116f8f9

webAuth = webAuth.presentationWindow(window)

We already have right examples in docs

[NandanPrabhu]

lets add unit tests to ContentViewModel to check if webauth works correctly on conversion to struct