feat(helexa-acp): repair malformed tool calls and render failures as cards

Two related fixes for cases where Qwen3 sometimes emits slightly-off
JSON inside <tool_call> blocks:

1. JSON repair pass in qwen3::parse_tool_call_body — strip up to
   three trailing extra `}` characters (model overshoots its closing
   braces), and hoist `name` out of `arguments` when it lands
   nested instead of as a sibling. Both observed in the field; both
   trivially repairable; both now dispatch as normal tool calls
   instead of falling back to the malformed path.

2. New CompletionEvent::MalformedToolCall variant for the cases
   repair can't fix. decode_stream now emits it instead of wrapping
   the raw body in a TextDelta, and agent.rs surfaces each one as
   a Failed SessionUpdate::ToolCall card (so Zed renders it as a
   structured failure UI element rather than dumping the body
   inline) plus a synthetic tool-call/tool-result history pair so
   the model gets clear feedback for self-correction on the next
   round.

Empty <tool_call></tool_call> blocks are now a no-op too (no
Malformed event), matching the existing empty-<think> behaviour.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/helexa-acp/src/provider/mod.rs

@@ -163,6 +163,14 @@ pub enum CompletionEvent {
     /// the bytes by `index` until the call's arguments are complete.
     #[allow(dead_code)]
     ToolCallArgsDelta { index: usize, args_delta: String },
+    /// A `<tool_call>` block whose JSON couldn't be parsed even with
+    /// the qwen3 module's repair attempts. The agent surfaces this
+    /// as a Failed `SessionUpdate::ToolCall` card with the raw body
+    /// visible (so the editor renders structured failure UI rather
+    /// than dumping the body inline in the message pane), and feeds
+    /// a synthetic tool-error message back into history so the
+    /// model can self-correct on the next round.
+    MalformedToolCall { raw: String },
     /// Stream finished. Carries the upstream `finish_reason` if it
     /// gave one (`"stop"`, `"length"`, `"tool_calls"`, …).
     Finish { reason: Option<String> },