fix: fast-fail on_load_session when thread ID is missing or wrong

When the UI sends a legacy session ID (e.g. "20260415_1") instead of a
thread UUID, on_load_session previously hung indefinitely because
get_thread returned an error that was silently swallowed or never
reached the client. This caused a 30-second frontend timeout and a
blank conversation view.

Now the handler checks whether the requested ID corresponds to a known
session in the sessions table, and returns an immediate, diagnostic
error explaining what went wrong:
- "Session X has linked thread Y, but was sent as the session_id
  instead of the thread UUID" — tells the engineer the UI needs to
  send the thread UUID instead.
- "Session X exists but has no linked thread" — the session was not
  fully created via ACP (thread creation failed or was skipped).
- "Session not found" — the ID doesn't match anything.

Also bumps tauri-plugin-dialog to "2" to fix a version mismatch
warning with @tauri-apps/plugin-dialog v2.7.

Signed-off-by: morgmart <98432065+morgmart@users.noreply.github.com>

crates/goose-acp/src/server.rs

@@ -1473,14 +1473,37 @@ impl GooseAcpAgent {
         // The ACP session_id IS the thread ID.
         let thread_id = args.session_id.0.to_string();
 
-        let thread = self
-            .thread_manager
-            .get_thread(&thread_id)
-            .await
-            .map_err(|_| {
-                sacp::Error::resource_not_found(Some(thread_id.clone()))
-                    .data(format!("Session not found: {}", thread_id))
-            })?;
+        let thread = match self.thread_manager.get_thread(&thread_id).await {
+            Ok(thread) => thread,
+            Err(_) => {
+                // Check if the requested ID is a legacy session ID (e.g.
+                // "20260415_1") that was sent instead of a thread UUID.
+                // Return a diagnostic error so the caller knows what happened.
+                if let Ok(session) = self
+                    .session_manager
+                    .get_session(&thread_id, false)
+                    .await
+                {
+                    let detail = match &session.thread_id {
+                        Some(tid) => format!(
+                            "Session {} has linked thread {}, but was sent as the \
+                             session_id instead of the thread UUID",
+                            thread_id, tid
+                        ),
+                        None => format!(
+                            "Session {} exists but has no linked thread — \
+                             it may not have been fully created via ACP",
+                            thread_id
+                        ),
+                    };
+                    return Err(sacp::Error::resource_not_found(Some(thread_id)).data(detail));
+                }
+                return Err(
+                    sacp::Error::resource_not_found(Some(thread_id.clone()))
+                        .data(format!("Session not found: {}", thread_id)),
+                );
+            }
+        };
 
         // Reuse the thread's current internal session so the agent retains
         // conversation context (compaction state, full message history, etc.).

ui/goose2/src-tauri/Cargo.toml

@@ -20,7 +20,7 @@ tauri-build = { version = "2", features = [] }
 tauri = { version = "2", features = ["protocol-asset"] }
 tauri-plugin-app-test-driver = { path = "plugins/app-test-driver" }
 tauri-plugin-opener = "2"
-tauri-plugin-dialog = ">=2,<2.7"
+tauri-plugin-dialog = "2"
 tauri-plugin-window-state = "2"
 tauri-plugin-log = "2"
 serde = { version = "1", features = ["derive"] }