fix: remove tool call ID truncation and improve DeepSeek reasoning handling

The 40-character truncation of tool call IDs in helper.go caused collisions
when models (like deepseek-v4-flash) generated longer IDs, leading to
"Duplicate value for 'tool_call_id'" errors. Removed the limit to allow
full unique IDs.

DeepSeek: updated reasoning_content injection to use an empty string
instead of a space, better matching provider expectations for history.

Improved API error reporting across all providers by capturing raw body
content when response parsing fails or returns empty strings.

internal/providers/gemini.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"strings"
 	"time"
 
@@ -125,7 +126,13 @@ func (p *GeminiProvider) ImageGeneration(ctx context.Context, req *models.ImageG
 	}
 
 	if !resp.IsSuccess() {
-		return nil, fmt.Errorf("Gemini Imagen API error (%d): %s", resp.StatusCode(), resp.String())
+		msg := resp.String()
+		if msg == "" {
+			if body, err := io.ReadAll(resp.RawBody()); err == nil {
+				msg = string(body)
+			}
+		}
+		return nil, fmt.Errorf("Gemini Imagen API error (%d): %s", resp.StatusCode(), msg)
 	}
 
 	// Parse Imagen response
@@ -363,11 +370,17 @@ func (p *GeminiProvider) ChatCompletion(ctx context.Context, req *models.Unified
 	}
 
 	if !resp.IsSuccess() {
-		fmt.Printf("[Gemini] API Error %d: %s\n", resp.StatusCode(), resp.String())
+		msg := resp.String()
+		if msg == "" {
+			if body, err := io.ReadAll(resp.RawBody()); err == nil {
+				msg = string(body)
+			}
+		}
+		fmt.Printf("[Gemini] API Error %d: %s\n", resp.StatusCode(), msg)
 		// Also log the request body for debugging (careful with API keys if logged elsewhere)
 		reqJSON, _ := json.Marshal(body)
 		fmt.Printf("[Gemini] Request Body: %s\n", string(reqJSON))
-		return nil, fmt.Errorf("Gemini API error (%d): %s", resp.StatusCode(), resp.String())
+		return nil, fmt.Errorf("Gemini API error (%d): %s", resp.StatusCode(), msg)
 	}
 
 	// Parse Gemini response and convert to OpenAI format
@@ -599,7 +612,13 @@ func (p *GeminiProvider) ChatCompletionStream(ctx context.Context, req *models.U
 	}
 
 	if !resp.IsSuccess() {
-		return nil, fmt.Errorf("Gemini API error (%d): %s", resp.StatusCode(), resp.String())
+		msg := resp.String()
+		if msg == "" {
+			if body, err := io.ReadAll(resp.RawBody()); err == nil {
+				msg = string(body)
+			}
+		}
+		return nil, fmt.Errorf("Gemini API error (%d): %s", resp.StatusCode(), msg)
 	}
 
 	ch := make(chan *models.ChatCompletionStreamResponse)