refactor(chat_router): Extract prompt builder functions (Step 5)

- Extracted prompt building functions to backend/api/handlers/prompt_builder.py:
  - get_validator_info_for_prompt()
  - build_prompt_context_from_chat_request()
  - truncate_user_message()
  - get_transparency_disclaimer()
  - build_minimal_philosophical_prompt()
  - format_conversation_history()
  - calculate_confidence_score()

- Added ValidatorInfo class to chat_config.py for validator defaults

- Updated chat_router.py to import and use new prompt builder functions

- Created comprehensive unit tests for prompt_builder.py (26 test cases)

- Reduced chat_router.py from ~8,983 LOC to 7,864 LOC (~1,119 lines removed)

This is Step 5 of the 10-step refactoring plan to reduce chat_router.py complexity.

Author: anhmtk

backend/api/config/chat_config.py

@@ -82,6 +82,14 @@ class FPSConfig:
     EXPLICIT_FAKE_ENTITIES: List[str] = None  # Will be initialized in __post_init__
 
 
+@dataclass
+class ValidatorInfo:
+    """Validator information defaults."""
+    DEFAULT_VI: str = "19 validators total, chia thành 7 lớp (layers)"
+    DEFAULT_EN: str = "19 validators total, organized into 7 layers"
+    DEFAULT_LAYERS: str = "7 layers"
+
+
 @dataclass
 class ChatConfig:
     """Main configuration class aggregating all config sections."""
@@ -94,6 +102,7 @@ class ChatConfig:
     cache: CacheConfig = None
     validation: ValidationConfig = None
     fps: FPSConfig = None
+    validator_info: ValidatorInfo = None
 
     def __post_init__(self):
         """Initialize nested config objects."""
@@ -116,6 +125,8 @@ def __post_init__(self):
             self.fps.EXPLICIT_FAKE_ENTITIES = [
                 "veridian", "lumeria", "emerald", "daxonia"
             ]
+        if self.validator_info is None:
+            self.validator_info = ValidatorInfo()
 
 
 # Global config instance (singleton pattern)