fix: Gemini provider compatibility with Gemini 3 models

- Strip additionalProperties and $schema from tool parameter schemas
  (Gemini API rejects these unsupported JSON Schema fields)
- Add thoughtSignature to functionCall parts for multi-turn tool calling
  (Gemini 3 models require thought signatures on function call history)
- Ensure functionResponse.response is always a JSON object, never an
  array or primitive (wrap non-objects in {"result": ...})
- Resolve tool names from tool_call_id or positional index when the
  tool message doesn't carry a name field
- Add functionCall parsing in streaming responses (emitGeminiChunk)
  to properly relay tool calls via SSE
- Add error logging for Gemini stream failures with request body dump
- Add unit tests for schema cleaning and streaming tool call emission

internal/providers/helpers.go

@@ -409,8 +409,12 @@ type geminiStreamChunk struct {
 	Candidates []struct {
 		Content struct {
 			Parts []struct {
-				Text    string `json:"text,omitempty"`
-				Thought string `json:"thought,omitempty"`
+				Text         string `json:"text,omitempty"`
+				Thought      string `json:"thought,omitempty"`
+				FunctionCall *struct {
+					Name string          `json:"name"`
+					Args json.RawMessage `json:"args"`
+				} `json:"functionCall,omitempty"`
 			} `json:"parts"`
 		} `json:"content"`
 		FinishReason string `json:"finishReason"`
@@ -433,6 +437,7 @@ func emitGeminiChunk(ch chan<- *models.ChatCompletionStreamResponse, chunk *gemi
 	content := ""
 	var reasoning *string
 	var finishReason *string
+	var toolCalls []models.ToolCallDelta
 	if len(chunk.Candidates) > 0 {
 		for _, p := range chunk.Candidates[0].Content.Parts {
 			if p.Text != "" {
@@ -444,8 +449,26 @@ func emitGeminiChunk(ch chan<- *models.ChatCompletionStreamResponse, chunk *gemi
 				}
 				*reasoning += p.Thought
 			}
+			if p.FunctionCall != nil {
+				name := p.FunctionCall.Name
+				args := string(p.FunctionCall.Args)
+				tcID := fmt.Sprintf("call_%s", name)
+				tcType := "function"
+				toolCalls = append(toolCalls, models.ToolCallDelta{
+					Index: uint32(len(toolCalls)),
+					ID:    &tcID,
+					Type:  &tcType,
+					Function: &models.FunctionCallDelta{
+						Name:      &name,
+						Arguments: &args,
+					},
+				})
+			}
 		}
 		fr := strings.ToLower(chunk.Candidates[0].FinishReason)
+		if len(toolCalls) > 0 && fr == "" {
+			fr = "tool_calls"
+		}
 		finishReason = &fr
 	}
 
@@ -460,6 +483,7 @@ func emitGeminiChunk(ch chan<- *models.ChatCompletionStreamResponse, chunk *gemi
 				Delta: models.ChatStreamDelta{
 					Content:          &content,
 					ReasoningContent: reasoning,
+					ToolCalls:        toolCalls,
 				},
 				FinishReason: finishReason,
 			},