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/application/src/plugin/mod.rs

@@ -31,6 +31,8 @@ pub struct PluginContributionSummary {
     pub top_level_menus: usize,
     /// Menu item count.
     pub menu_items: usize,
+    /// Slash-command count.
+    pub slash_commands: usize,
     /// Layout count.
     pub layouts: usize,
     /// MCP server count.
@@ -42,6 +44,7 @@ impl From<&PluginContributionSet> for PluginContributionSummary {
         Self {
             top_level_menus: c.menus.len(),
             menu_items: c.menu_items.len(),
+            slash_commands: c.slash_commands.len(),
             layouts: c.layouts.len(),
             mcp_servers: c.mcp_servers.len(),
         }
@@ -3088,6 +3091,8 @@ struct RawContributes {
     #[serde(default)]
     menu_items: Vec<RawMenuItem>,
     #[serde(default)]
+    slash_commands: Vec<RawSlashCommand>,
+    #[serde(default)]
     layouts: Vec<RawLayout>,
     #[serde(default)]
     mcp_servers: Vec<RawMcpServer>,
@@ -3120,6 +3125,18 @@ struct RawMenuItem {
     when: Option<String>,
 }
 
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct RawSlashCommand {
+    name: String,
+    short_description: String,
+    command: String,
+    #[serde(default)]
+    requires_confirmation: bool,
+    #[serde(default)]
+    when: Option<String>,
+}
+
 #[derive(Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 struct RawLayout {
@@ -3283,6 +3300,31 @@ fn validate_contributes(raw: RawContributes) -> Result<PluginContributionSet, Pl
             })
         })
         .collect::<Result<Vec<_>, _>>()?;
+    let slash_commands = raw
+        .slash_commands
+        .into_iter()
+        .map(|s| {
+            insert(&s.name)?;
+            if !s.name.starts_with('/') {
+                return Err(PluginManifestError::Invalid(
+                    "slashCommands[].name must start with '/'".to_owned(),
+                ));
+            }
+            if s.short_description.trim().is_empty() {
+                return Err(PluginManifestError::Invalid(
+                    "slashCommands[].shortDescription is required".to_owned(),
+                ));
+            }
+            Ok(domain::PluginSlashCommandContribution {
+                name: s.name,
+                short_description: s.short_description,
+                command: domain::PluginCommandId::new(s.command)
+                    .map_err(|e| PluginManifestError::Invalid(e.to_string()))?,
+                requires_confirmation: s.requires_confirmation,
+                when: s.when,
+            })
+        })
+        .collect::<Result<Vec<_>, _>>()?;
     let layouts = raw
         .layouts
         .into_iter()
@@ -3353,6 +3395,7 @@ fn validate_contributes(raw: RawContributes) -> Result<PluginContributionSet, Pl
     Ok(PluginContributionSet {
         menus,
         menu_items,
+        slash_commands,
         layouts,
         mcp_servers,
     })
@@ -3423,6 +3466,7 @@ mod tests {
           "contributes": {
             "menus": [{"id":"dev.acme.menu","label":"Graph","topLevel":true}],
             "menuItems": [{"id":"dev.acme.open","targetMenuId":"panels","label":"Open","command":"dev.acme.open"}],
+            "slashCommands": [{"name":"/graph","shortDescription":"Open graph tools","command":"dev.acme.open","requiresConfirmation":true}],
             "layouts": [{"type":"dev.acme.layout","label":"Graph","component":"Graph"}],
             "mcpServers": [{"id":"dev.acme.mcp","displayName":"Tools","command":"servers/tool","transport":"stdio","autoStart":true}]
           }
@@ -3745,6 +3789,9 @@ mod tests {
         );
         assert_eq!(m.contributes.layouts.len(), 1);
         assert_eq!(m.contributes.mcp_servers.len(), 1);
+        assert_eq!(m.contributes.slash_commands.len(), 1);
+        assert_eq!(m.contributes.slash_commands[0].name, "/graph");
+        assert!(m.contributes.slash_commands[0].requires_confirmation);
     }
 
     #[test]
@@ -3809,6 +3856,21 @@ mod tests {
             .is_err());
     }
 
+    #[test]
+    fn rejects_slash_command_without_leading_slash() {
+        let mut value: serde_json::Value = serde_json::from_slice(&valid_manifest()).unwrap();
+        value["contributes"]["slashCommands"][0]["name"] = serde_json::json!("graph");
+        assert!(validator()
+            .validate(
+                &serde_json::to_vec(&value).unwrap(),
+                &domain::PluginPackageRef {
+                    plugin_id: None,
+                    root: "x".into()
+                }
+            )
+            .is_err());
+    }
+
     #[tokio::test]
     async fn runtime_catalog_excludes_disabled_and_pending_uninstall_plugins() {
         for state in [
@@ -3849,6 +3911,7 @@ mod tests {
                 domain::PluginCapability::Tooling,
             ]
         );
+        assert_eq!(catalog.plugins[0].contributes.slash_commands.len(), 1);
     }
 
     #[tokio::test]