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>

UnitTests/Sources/CurrentValuePublisherTests.swift

@@ -0,0 +1,48 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+// Please see LICENSE files in the repository root for full details.
+//
+
+import Combine
+@testable import ElementX
+import Testing
+
+struct CurrentValuePublisherTests {
+    @Test
+    func publisherReplaysCurrentValueAndDeliversSubsequentOnes() {
+        let subject = CurrentValueSubject<Int, Never>(1)
+        var received = [Int]()
+        let cancellable = subject.asCurrentValuePublisher().sink { received.append($0) }
+        subject.send(2)
+        subject.send(3)
+        #expect(received == [1, 2, 3])
+        cancellable.cancel()
+    }
+    
+    @Test
+    func mappedPublisherChainedInlineDeliversSubsequentValues() {
+        let subject = CurrentValueSubject<Int, Never>(1)
+        var received = [Int]()
+        // The mapped publisher is intentionally chained without being stored: the struct is
+        // discarded mid-chain, which used to cancel an eager map's subscription and stop updates.
+        let cancellable = subject.asCurrentValuePublisher()
+            .map { $0 * 2 }
+            .removeDuplicates()
+            .sink { received.append($0) }
+        subject.send(2)
+        subject.send(3)
+        #expect(received == [2, 4, 6])
+        cancellable.cancel()
+    }
+    
+    @Test
+    func mappedPublisherValueStaysCurrent() {
+        let subject = CurrentValueSubject<Int, Never>(1)
+        let mapped = subject.asCurrentValuePublisher().map { $0 * 2 }
+        #expect(mapped.value == 2)
+        subject.send(5)
+        #expect(mapped.value == 10)
+    }
+}