feat(plugins): seam slash-commands contribuées par plugin via callback — #165 (QA verte)

Permet à un plugin de contribuer des commandes slash adossées à une callback,
transitant par le registry/contrat unifié (#162) :

- sdk/IdeaSDK: manifeste contributes.slashCommands (déclaratif) — pointer bumpé.
- domain: source Plugin étendue + effet PluginCallback (la commande ne
  s'exécute pas directement : elle renvoie une référence à la callback du plugin).
- application: registration des commandes plugin dans le registry + exécution
  renvoyant l'effet PluginCallback ; métadonnées UI suffisantes pour l'autocomplete.
- backend + app-tauri: DTO + commandes Tauri listant/invoquant les commandes plugin.
- frontend (contrat): types adapters/domain/ports/mock pour pluginCallback et
  l'invocation avec arguments.

L'exécution effective de la callback côté runtime UI est livrée par #166.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/domain/src/slash_command.rs

@@ -17,6 +17,7 @@ pub enum SlashCommandSource {
     /// Contributed by a plugin.
     Plugin {
         /// Opaque plugin id.
+        #[serde(rename = "pluginId")]
         plugin_id: String,
     },
 }
@@ -54,6 +55,16 @@ pub enum NativeSlashCommand {
     Profile,
 }
 
+/// Plugin callback identity for a contributed slash command.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct PluginSlashCommandCallback {
+    /// Opaque plugin id.
+    pub plugin_id: String,
+    /// SDK command callback id registered by the plugin.
+    pub command_id: String,
+}
+
 /// UI-facing slash-command metadata.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
@@ -71,6 +82,9 @@ pub struct SlashCommand {
     /// Built-in command discriminator when `source == native`.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub native: Option<NativeSlashCommand>,
+    /// Plugin callback identity when `source == plugin`.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub plugin: Option<PluginSlashCommandCallback>,
 }
 
 impl SlashCommand {
@@ -86,6 +100,7 @@ impl SlashCommand {
         availability: SlashCommandAvailability,
         source: SlashCommandSource,
         native: Option<NativeSlashCommand>,
+        plugin: Option<PluginSlashCommandCallback>,
     ) -> Result<Self, DomainError> {
         let name = name.into();
         let short_description = short_description.into();
@@ -103,6 +118,7 @@ impl SlashCommand {
             availability,
             source,
             native,
+            plugin,
         })
     }
 }
@@ -118,6 +134,7 @@ pub fn native_slash_commands() -> Vec<SlashCommand> {
             SlashCommandAvailability::Available,
             SlashCommandSource::Native,
             Some(NativeSlashCommand::Help),
+            None,
         )
         .expect("valid native slash command"),
         SlashCommand::new(
@@ -127,6 +144,7 @@ pub fn native_slash_commands() -> Vec<SlashCommand> {
             SlashCommandAvailability::Available,
             SlashCommandSource::Native,
             Some(NativeSlashCommand::Clean),
+            None,
         )
         .expect("valid native slash command"),
         SlashCommand::new(
@@ -136,11 +154,41 @@ pub fn native_slash_commands() -> Vec<SlashCommand> {
             SlashCommandAvailability::Available,
             SlashCommandSource::Native,
             Some(NativeSlashCommand::Profile),
+            None,
         )
         .expect("valid native slash command"),
     ]
 }
 
+/// Builds validated metadata for a plugin-contributed slash command.
+pub fn plugin_slash_command(
+    plugin_id: impl Into<String>,
+    command_id: impl Into<String>,
+    name: impl Into<String>,
+    short_description: impl Into<String>,
+    requires_confirmation: bool,
+    availability: SlashCommandAvailability,
+) -> Result<SlashCommand, DomainError> {
+    let plugin_id = plugin_id.into();
+    let command_id = command_id.into();
+    crate::validation::non_empty(&plugin_id, "slash_command.plugin_id")?;
+    crate::validation::non_empty(&command_id, "slash_command.command_id")?;
+    SlashCommand::new(
+        name,
+        short_description,
+        requires_confirmation,
+        availability,
+        SlashCommandSource::Plugin {
+            plugin_id: plugin_id.clone(),
+        },
+        None,
+        Some(PluginSlashCommandCallback {
+            plugin_id,
+            command_id,
+        }),
+    )
+}
+
 /// Filters commands by a user-entered slash prefix.
 ///
 /// Blank prefixes return every command. Prefixes without a leading `/` are
@@ -180,6 +228,7 @@ mod tests {
         assert!(commands[1].availability.is_available());
         assert!(commands[2].availability.is_available());
         assert!(commands[2].requires_confirmation);
+        assert!(commands.iter().all(|c| c.plugin.is_none()));
     }
 
     #[test]
@@ -196,4 +245,24 @@ mod tests {
             .collect::<Vec<_>>();
         assert_eq!(names, vec!["/profile"]);
     }
+
+    #[test]
+    fn plugin_command_carries_callback_identity() {
+        let command = plugin_slash_command(
+            "dev.acme.plugin",
+            "dev.acme.plugin.explain",
+            "/explain",
+            "Explain selection",
+            true,
+            SlashCommandAvailability::Available,
+        )
+        .unwrap();
+
+        assert_eq!(command.name, "/explain");
+        assert!(command.native.is_none());
+        assert_eq!(
+            command.plugin.unwrap().command_id,
+            "dev.acme.plugin.explain"
+        );
+    }
 }