refactor: extract token budget constants for thinking levels with validation bounds

## CHANGES

- Extract hardcoded token values into named constants
- Add comprehensive documentation for token budget purposes
- Implement token validation bounds (1-10000) in parsing
- Replace magic numbers with semantic constant references
- Improve code maintainability through constant extraction

internal/domain/thinking.go

@@ -10,9 +10,25 @@ const (
 	ThinkingHigh   ThinkingLevel = "high"
 )
 
+// ThinkingBudgets defines standardized token budgets for reasoning-enabled models.
+// The map assigns a maximum token count to each ThinkingLevel, representing the
+// amount of context or computation that can be used for reasoning at that level.
+// These values (e.g., 1024 for low, 2048 for medium, 4096 for high) are used to
+// Token budget constants for each ThinkingLevel.
+// These values are chosen to align with typical context window sizes for LLMs at different reasoning levels.
+// Adjust these if model capabilities change.
+const (
+	// TokenBudgetLow is suitable for basic reasoning or smaller models (e.g., 1k context window).
+	TokenBudgetLow int64 = 1024
+	// TokenBudgetMedium is suitable for intermediate reasoning or mid-sized models (e.g., 2k context window).
+	TokenBudgetMedium int64 = 2048
+	// TokenBudgetHigh is suitable for advanced reasoning or large models (e.g., 4k context window).
+	TokenBudgetHigh int64 = 4096
+)
+
 // ThinkingBudgets defines standardized token budgets for reasoning-enabled models.
 var ThinkingBudgets = map[ThinkingLevel]int64{
-	ThinkingLow:    1024,
-	ThinkingMedium: 2048,
-	ThinkingHigh:   4096,
+	ThinkingLow:    TokenBudgetLow,
+	ThinkingMedium: TokenBudgetMedium,
+	ThinkingHigh:   TokenBudgetHigh,
 }

internal/plugins/ai/anthropic/anthropic.go

@@ -167,7 +167,9 @@ func parseThinking(level domain.ThinkingLevel) (anthropic.ThinkingConfigParamUni
 		}
 	default:
 		if tokens, err := strconv.ParseInt(lower, 10, 64); err == nil {
-			return anthropic.ThinkingConfigParamOfEnabled(tokens), true
+			if tokens >= 1 && tokens <= 10000 {
+				return anthropic.ThinkingConfigParamOfEnabled(tokens), true
+			}
 		}
 	}
 	return anthropic.ThinkingConfigParamUnion{}, false