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/openai_chat.rs

@@ -758,10 +758,8 @@ where
                                         });
                                     }
                                     crate::qwen3::ParserEvent::Malformed { raw } => {
-                                        tracing::warn!(raw = %raw, "qwen3: malformed <tool_call> block; passing through as text");
-                                        yield Ok(CompletionEvent::TextDelta(format!(
-                                            "<tool_call>{raw}</tool_call>"
-                                        )));
+                                        tracing::warn!(raw = %raw, "qwen3: malformed <tool_call> block; surfacing as Failed tool card");
+                                        yield Ok(CompletionEvent::MalformedToolCall { raw });
                                     }
                                 }
                             }
@@ -842,6 +840,7 @@ where
                                     }
                                     crate::qwen3::ParserEvent::Malformed { raw } => {
                                         tracing::warn!(raw = %raw, "qwen3: unterminated <tool_call> at stream end");
+                                        yield Ok(CompletionEvent::MalformedToolCall { raw });
                                     }
                                 }
                             }