feat(context): implement model adapters for Claude and OpenAI (#83)

Phase 5 of Context Management Engine - Model Adapters:

- Add ModelAdapter abstract base class with model matching
- Add DefaultAdapter for unknown models (plain text)
- Add ClaudeAdapter with XML-based formatting:
  - <system_instructions> for system context
  - <reference_documents>/<document> for knowledge
  - <conversation_history>/<message> for chat
  - <tool_results>/<tool_result> for tool outputs
  - XML escaping for special characters
- Add OpenAIAdapter with markdown formatting:
  - ## headers for sections
  - ### Source headers for documents
  - **ROLE** bold labels for conversation
  - Code blocks for tool outputs
- Add get_adapter() factory function for model selection

Tests: 33 new tests, 256 total context tests passing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/context/adapters/base.py

@@ -0,0 +1,178 @@
+"""
+Base Model Adapter.
+
+Abstract base class for model-specific context formatting.
+"""
+
+from abc import ABC, abstractmethod
+from typing import Any
+
+from ..types import BaseContext, ContextType
+
+
+class ModelAdapter(ABC):
+    """
+    Abstract base adapter for model-specific context formatting.
+
+    Each adapter knows how to format contexts for optimal
+    understanding by a specific LLM family (Claude, OpenAI, etc.).
+    """
+
+    # Model name patterns this adapter handles
+    MODEL_PATTERNS: list[str] = []
+
+    @classmethod
+    def matches_model(cls, model: str) -> bool:
+        """
+        Check if this adapter handles the given model.
+
+        Args:
+            model: Model name to check
+
+        Returns:
+            True if this adapter handles the model
+        """
+        model_lower = model.lower()
+        return any(pattern in model_lower for pattern in cls.MODEL_PATTERNS)
+
+    @abstractmethod
+    def format(
+        self,
+        contexts: list[BaseContext],
+        **kwargs: Any,
+    ) -> str:
+        """
+        Format contexts for the target model.
+
+        Args:
+            contexts: List of contexts to format
+            **kwargs: Additional formatting options
+
+        Returns:
+            Formatted context string
+        """
+        ...
+
+    @abstractmethod
+    def format_type(
+        self,
+        contexts: list[BaseContext],
+        context_type: ContextType,
+        **kwargs: Any,
+    ) -> str:
+        """
+        Format contexts of a specific type.
+
+        Args:
+            contexts: List of contexts of the same type
+            context_type: The type of contexts
+            **kwargs: Additional formatting options
+
+        Returns:
+            Formatted string for this context type
+        """
+        ...
+
+    def get_type_order(self) -> list[ContextType]:
+        """
+        Get the preferred order of context types.
+
+        Returns:
+            List of context types in preferred order
+        """
+        return [
+            ContextType.SYSTEM,
+            ContextType.TASK,
+            ContextType.KNOWLEDGE,
+            ContextType.CONVERSATION,
+            ContextType.TOOL,
+        ]
+
+    def group_by_type(
+        self, contexts: list[BaseContext]
+    ) -> dict[ContextType, list[BaseContext]]:
+        """
+        Group contexts by their type.
+
+        Args:
+            contexts: List of contexts to group
+
+        Returns:
+            Dictionary mapping context type to list of contexts
+        """
+        by_type: dict[ContextType, list[BaseContext]] = {}
+        for context in contexts:
+            ct = context.get_type()
+            if ct not in by_type:
+                by_type[ct] = []
+            by_type[ct].append(context)
+        return by_type
+
+    def get_separator(self) -> str:
+        """
+        Get the separator between context sections.
+
+        Returns:
+            Separator string
+        """
+        return "\n\n"
+
+
+class DefaultAdapter(ModelAdapter):
+    """
+    Default adapter for unknown models.
+
+    Uses simple plain-text formatting with minimal structure.
+    """
+
+    MODEL_PATTERNS: list[str] = []  # Fallback adapter
+
+    @classmethod
+    def matches_model(cls, model: str) -> bool:
+        """Always returns True as fallback."""
+        return True
+
+    def format(
+        self,
+        contexts: list[BaseContext],
+        **kwargs: Any,
+    ) -> str:
+        """Format contexts as plain text."""
+        if not contexts:
+            return ""
+
+        by_type = self.group_by_type(contexts)
+        parts: list[str] = []
+
+        for ct in self.get_type_order():
+            if ct in by_type:
+                formatted = self.format_type(by_type[ct], ct, **kwargs)
+                if formatted:
+                    parts.append(formatted)
+
+        return self.get_separator().join(parts)
+
+    def format_type(
+        self,
+        contexts: list[BaseContext],
+        context_type: ContextType,
+        **kwargs: Any,
+    ) -> str:
+        """Format contexts of a type as plain text."""
+        if not contexts:
+            return ""
+
+        content = "\n\n".join(c.content for c in contexts)
+
+        if context_type == ContextType.SYSTEM:
+            return content
+        elif context_type == ContextType.TASK:
+            return f"Task:\n{content}"
+        elif context_type == ContextType.KNOWLEDGE:
+            return f"Reference Information:\n{content}"
+        elif context_type == ContextType.CONVERSATION:
+            return f"Previous Conversation:\n{content}"
+        elif context_type == ContextType.TOOL:
+            return f"Tool Results:\n{content}"
+
+        return content