feat(kiro): enhance thinking support and fix truncation issues

- **Thinking Support**:
    - Enabled thinking support for all Kiro Claude models, including Haiku 4.5 and agentic variants.
    - Updated `model_definitions.go` with thinking configuration (Min: 1024, Max: 32000, ZeroAllowed: true).
    - Fixed `extended_thinking` field names in `model_registry.go` (from `min_budget`/`max_budget` to `min`/`max`) to comply with Claude API specs, enabling thinking control in clients like Claude Code.

- **Kiro Executor Fixes**:
    - Fixed `budget_tokens` handling: explicitly disable thinking when budget is 0 or negative.
    - Removed aggressive duplicate content filtering logic that caused truncation/data loss.
    - Enhanced thinking tag parsing with `extractThinkingFromContent` to correctly handle interleaved thinking/text blocks.
    - Added EOF handling to flush pending thinking tag characters, preventing data loss at stream end.

- **Performance**:
    - Optimized Claude stream handler (v6.2) with reduced buffer size (4KB) and faster flush interval (50ms) to minimize latency and prevent timeouts.

internal/registry/model_definitions.go

@@ -895,6 +895,7 @@ func GetKiroModels() []*ModelInfo {
 			Description:         "Claude Opus 4.5 via Kiro (2.2x credit)",
 			ContextLength:       200000,
 			MaxCompletionTokens: 64000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 32000, ZeroAllowed: true, DynamicAllowed: true},
 		},
 		{
 			ID:                  "kiro-claude-sonnet-4-5",
@@ -906,6 +907,7 @@ func GetKiroModels() []*ModelInfo {
 			Description:         "Claude Sonnet 4.5 via Kiro (1.3x credit)",
 			ContextLength:       200000,
 			MaxCompletionTokens: 64000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 32000, ZeroAllowed: true, DynamicAllowed: true},
 		},
 		{
 			ID:                  "kiro-claude-sonnet-4",
@@ -917,6 +919,7 @@ func GetKiroModels() []*ModelInfo {
 			Description:         "Claude Sonnet 4 via Kiro (1.3x credit)",
 			ContextLength:       200000,
 			MaxCompletionTokens: 64000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 32000, ZeroAllowed: true, DynamicAllowed: true},
 		},
 		{
 			ID:                  "kiro-claude-haiku-4-5",
@@ -928,6 +931,7 @@ func GetKiroModels() []*ModelInfo {
 			Description:         "Claude Haiku 4.5 via Kiro (0.4x credit)",
 			ContextLength:       200000,
 			MaxCompletionTokens: 64000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 32000, ZeroAllowed: true, DynamicAllowed: true},
 		},
 		// --- Agentic Variants (Optimized for coding agents with chunked writes) ---
 		{
@@ -940,6 +944,7 @@ func GetKiroModels() []*ModelInfo {
 			Description:         "Claude Opus 4.5 optimized for coding agents (chunked writes)",
 			ContextLength:       200000,
 			MaxCompletionTokens: 64000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 32000, ZeroAllowed: true, DynamicAllowed: true},
 		},
 		{
 			ID:                  "kiro-claude-sonnet-4-5-agentic",
@@ -951,6 +956,7 @@ func GetKiroModels() []*ModelInfo {
 			Description:         "Claude Sonnet 4.5 optimized for coding agents (chunked writes)",
 			ContextLength:       200000,
 			MaxCompletionTokens: 64000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 32000, ZeroAllowed: true, DynamicAllowed: true},
 		},
 		{
 			ID:                  "kiro-claude-sonnet-4-agentic",
@@ -962,6 +968,7 @@ func GetKiroModels() []*ModelInfo {
 			Description:         "Claude Sonnet 4 optimized for coding agents (chunked writes)",
 			ContextLength:       200000,
 			MaxCompletionTokens: 64000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 32000, ZeroAllowed: true, DynamicAllowed: true},
 		},
 		{
 			ID:                  "kiro-claude-haiku-4-5-agentic",
@@ -973,6 +980,7 @@ func GetKiroModels() []*ModelInfo {
 			Description:         "Claude Haiku 4.5 optimized for coding agents (chunked writes)",
 			ContextLength:       200000,
 			MaxCompletionTokens: 64000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 32000, ZeroAllowed: true, DynamicAllowed: true},
 		},
 	}
 }

internal/registry/model_registry.go

@@ -748,7 +748,8 @@ func (r *ModelRegistry) convertModelToMap(model *ModelInfo, handlerType string)
 		}
 		return result
 
-	case "claude":
+	case "claude", "kiro", "antigravity":
+		// Claude, Kiro, and Antigravity all use Claude-compatible format for Claude Code client
 		result := map[string]any{
 			"id":       model.ID,
 			"object":   "model",
@@ -763,6 +764,19 @@ func (r *ModelRegistry) convertModelToMap(model *ModelInfo, handlerType string)
 		if model.DisplayName != "" {
 			result["display_name"] = model.DisplayName
 		}
+		// Add thinking support for Claude Code client
+		// Claude Code checks for "thinking" field (simple boolean) to enable tab toggle
+		// Also add "extended_thinking" for detailed budget info
+		if model.Thinking != nil {
+			result["thinking"] = true
+			result["extended_thinking"] = map[string]any{
+				"supported":       true,
+				"min":             model.Thinking.Min,
+				"max":             model.Thinking.Max,
+				"zero_allowed":    model.Thinking.ZeroAllowed,
+				"dynamic_allowed": model.Thinking.DynamicAllowed,
+			}
+		}
 		return result
 
 	case "gemini":