Fix CurrentValuePublisher.map dropping updates when the mapped publisher is chained inline (#5870)

* Fix CurrentValuePublisher.map dropping updates when the mapped publisher is chained inline

The eager implementation bridged through a second CurrentValueSubject fed by
a sink whose AnyCancellable was stored on the returned struct. Combine
consumes publisher structs when building a subscription, so chaining the
mapped publisher inline (e.g. `publisher.map { … }.removeDuplicates(…)
.sink { … }`, as RoomScreenViewModel and RoomDetailsScreenViewModel do for
knock requests) dropped the only copy holding that cancellable: the feeding
sink was cancelled and subscribers only ever received the initial value.

In practice this meant a room admin viewing a knockable room never saw the
"wants to join" banner (or a live "Requests to join" badge) for a knock
arriving while the room was open — it only appeared when the knock predated
opening the room. Caught by a live cross-client knocking test; the
double-execution of the transform at subscription time (eager transform +
current-value replay) was the giveaway.

Make map lazy instead: keep the transformed AnyPublisher and a value
provider, so subscribers attach straight through to the source subject (which
replays its current value itself) and no keep-alive is needed. Adds a
regression test chaining a mapped publisher inline.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* swiftformat

* better docs

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Mauro Romito <mauro.romito@element.io>

ElementX/Sources/Other/CurrentValuePublisher.swift

@@ -13,33 +13,41 @@ import Combine
 ///
 /// `CurrentValueSubject` is documented as thread-safe but is not formally `Sendable`, hence `@unchecked`.
 nonisolated struct CurrentValuePublisher<Output, Failure: Error>: Publisher, @unchecked Sendable {
-    private let subject: CurrentValueSubject<Output, Failure>
-    /// Retains the upstream subscription feeding ``subject`` when this publisher is derived via ``map(_:)``.
-    private let cancellable: AnyCancellable?
+    private let upstream: AnyPublisher<Output, Failure>
+    private let valueProvider: () -> Output
     
-    init(_ subject: CurrentValueSubject<Output, Failure>, cancellable: AnyCancellable? = nil) {
-        self.subject = subject
-        self.cancellable = cancellable
+    init(_ subject: CurrentValueSubject<Output, Failure>) {
+        upstream = subject.eraseToAnyPublisher()
+        valueProvider = { subject.value }
     }
     
     init(_ value: Output) {
         self.init(CurrentValueSubject(value))
     }
     
+    private init(upstream: AnyPublisher<Output, Failure>, valueProvider: @escaping () -> Output) {
+        self.upstream = upstream
+        self.valueProvider = valueProvider
+    }
+    
     func receive<S: Subscriber>(subscriber: S) where Failure == S.Failure, Output == S.Input {
-        subject.receive(subscriber: subscriber)
+        upstream.receive(subscriber: subscriber)
     }
     
     var value: Output {
-        subject.value
+        valueProvider()
     }
 }
 
 nonisolated extension CurrentValuePublisher where Failure == Never {
+    /// Transforms the published values and ``value``.
+    ///
+    /// The transform runs lazily, once per subscriber and per ``value`` read. It must stay lazy:
+    /// this struct is discarded when the mapped publisher is chained inline, so anything eager
+    /// (a live subscription feeding a second subject) would be cancelled and stop the updates.
     func map<T>(_ transform: @escaping (Output) -> T) -> CurrentValuePublisher<T, Never> {
-        let subject = CurrentValueSubject<T, Never>(transform(value))
-        let cancellable = sink { subject.send(transform($0)) }
-        return .init(subject, cancellable: cancellable)
+        .init(upstream: upstream.map(transform).eraseToAnyPublisher(),
+              valueProvider: { transform(value) })
     }
 }