Add support for slash commands (under Feature Flag) (#6482)

* Add support for slash commands

* Update screenshots

* Rename module `slash` to `slashcommands`

* Rename `SlashCommand` to `SlashCommandService`

* Introduce MsgType in order to send text message with a different msgtype value.

* Format file and add parameter names, add default values and cleanup

* Add isSupported parameter to filter out unsupported yet commands.

* Slash commands: disable suggestions if the feature is disabled.

* Fix sending shrug command.

* Add missing test on SuggestionsProcessor

* Add tests on MessageComposerPresenter about slash command.

* Fix import ordering

* Add missing tests on CommandExecutor

* Add missing tests in MarkdownTextEditorStateTest

* Slash commands: Improve code when sending message with prefix.

* Slash commands: Add support for /unflip

---------

Co-authored-by: ElementBot <android@element.io>

libraries/slashcommands/test/build.gradle.kts

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 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.
+ */
+plugins {
+    id("io.element.android-library")
+}
+
+android {
+    namespace = "io.element.android.libraries.slashcommands.test"
+}
+
+dependencies {
+    implementation(projects.libraries.slashcommands.api)
+    implementation(projects.libraries.matrix.api)
+    implementation(projects.tests.testutils)
+}

libraries/slashcommands/test/src/main/kotlin/io/element/android/libraries/slashcommands/test/FakeSlashCommandService.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 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.
+ */
+
+package io.element.android.libraries.slashcommands.test
+
+import io.element.android.libraries.matrix.api.timeline.Timeline
+import io.element.android.libraries.slashcommands.api.SlashCommand
+import io.element.android.libraries.slashcommands.api.SlashCommandService
+import io.element.android.libraries.slashcommands.api.SlashCommandSuggestion
+import io.element.android.tests.testutils.lambda.lambdaError
+import io.element.android.tests.testutils.simulateLongTask
+
+class FakeSlashCommandService(
+    private val getSuggestionsResult: (String, Boolean) -> List<SlashCommandSuggestion> = { _, _ -> lambdaError() },
+    private val parseResult: (CharSequence, String?, Boolean) -> SlashCommand = { _, _, _ -> lambdaError() },
+    private val proceedSendMessageResult: (SlashCommand.SlashCommandSendMessage, Timeline) -> Result<Unit> = { _, _ -> lambdaError() },
+    private val proceedAdminResult: (SlashCommand.SlashCommandAdmin) -> Result<Unit> = { lambdaError() },
+) : SlashCommandService {
+    override suspend fun getSuggestions(text: String, isInThread: Boolean): List<SlashCommandSuggestion> = simulateLongTask {
+        getSuggestionsResult(text, isInThread)
+    }
+
+    override suspend fun parse(
+        textMessage: CharSequence,
+        formattedMessage: String?,
+        isInThreadTimeline: Boolean,
+    ): SlashCommand = simulateLongTask {
+        parseResult(textMessage, formattedMessage, isInThreadTimeline)
+    }
+
+    override suspend fun proceedSendMessage(
+        slashCommand: SlashCommand.SlashCommandSendMessage,
+        timeline: Timeline,
+    ): Result<Unit> = simulateLongTask {
+        proceedSendMessageResult(slashCommand, timeline)
+    }
+
+    override suspend fun proceedAdmin(slashCommand: SlashCommand.SlashCommandAdmin): Result<Unit> = simulateLongTask {
+        proceedAdminResult(slashCommand)
+    }
+}