fix: safe JSON parsing in generateChatResponse

[Priyanshu-byte-coder]

Summary

Closes #170

generateChatResponse() used unchecked casts (as List<dynamic>) and deep indexing (candidates[0]['content']['parts'][0]['text']) on Gemini API responses. Any deviation from the expected structure (rate limit, 500 error returning HTML, empty candidates, missing fields) would cause TypeError, RangeError, or NoSuchMethodError crashes.

Changes

lib/services/ai_service.dart — both the main chat block and the function-execution block:

1. jsonDecode wrapped in try/catch — returns controlled error if body isn't valid JSON
2. candidatesRaw is! List || isEmpty guard before indexing
3. contentRaw is! Map guard before accessing ['parts']
4. partsRaw is! List guard before iterating
5. part is Map check before containsKey('functionCall')
6. Null-safe text extraction via ?.toString() ?? ''

Before / After

// Before — crashes on malformed response
final candidates = responseData['candidates'] as List<dynamic>; // ❌ unsafe cast
return candidates[0]['content']['parts'][0]['text'] ?? '';       // ❌ unsafe indexing

// After — safe
final candidatesRaw = responseData['candidates'];
if (candidatesRaw is! List || candidatesRaw.isEmpty) {
  return {'type': 'error', 'content': 'No response generated'};
}

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