fix(gemini): override finish_reason to 'tool_calls' when tools are present

Gemini often reports 'STOP' even when tool calls are generated. To remain
OpenAI-compatible and ensure clients execute tools and continue, we must
report 'tool_calls' as the finish_reason when tools are present.

src/providers/gemini.rs

@@ -927,12 +927,18 @@ impl super::Provider for GeminiProvider {
                                 let tool_calls = if deltas.is_empty() { None } else { Some(deltas) };
 
                                 // Determine finish_reason
-                                let finish_reason = candidate.finish_reason.as_ref().map(|fr| {
+                                // STRATEGY: If we have tool calls, the finish_reason MUST be "tool_calls"
+                                // to comply with OpenAI-style expectations and ensure the client continues.
+                                let finish_reason = if tool_calls.is_some() {
+                                    Some("tool_calls".to_string())
+                                } else {
+                                    candidate.finish_reason.as_ref().map(|fr| {
                                         match fr.as_str() {
                                             "STOP" => "stop".to_string(),
                                             _ => fr.to_lowercase(),
                                         }
-                                });
+                                    })
+                                };
 
                                 // Avoid emitting completely empty chunks unless they carry usage.
                                 if !content.is_empty() || reasoning_content.is_some() || tool_calls.is_some() || stream_usage.is_some() {