feat: real-time tool call streaming indicator with blinking UI

- Add ToolParsingHint enum (Detected/Active/Complete) for UI feedback
- New UiWriter methods: print_tool_streaming_hint(), print_tool_streaming_active()
- Refactor ConsoleUiWriter state to use atomics in ParsingHintState
- Add tool_call_streaming field to CompletionChunk for provider hints
- Anthropic provider sends streaming hints when tool name detected
- New streaming helpers: make_tool_streaming_hint(), make_tool_streaming_active()

Parser improvements:
- Add is_json_invalidated() to detect false positive tool patterns
- Fix tool result poisoning when file contents contain partial JSON
- Unescaped newlines in strings or prose after JSON invalidates detection

User sees ' ● tool_name |' immediately when tool call starts streaming,
with blinking indicator while args are received.

crates/g3-providers/src/lib.rs

@@ -205,6 +205,8 @@ pub struct CompletionChunk {
     pub usage: Option<Usage>, // Add usage tracking for streaming
     /// Stop reason from the API (e.g., "end_turn", "max_tokens", "stop_sequence")
     pub stop_reason: Option<String>,
+    /// Tool call currently being streamed (name only, for UI hint)
+    pub tool_call_streaming: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]