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/claude.py

@@ -0,0 +1,178 @@
+"""
+Claude Model Adapter.
+
+Provides Claude-specific context formatting using XML tags
+which Claude models understand natively.
+"""
+
+from typing import Any
+
+from ..types import BaseContext, ContextType
+from .base import ModelAdapter
+
+
+class ClaudeAdapter(ModelAdapter):
+    """
+    Claude-specific context formatting adapter.
+
+    Claude models have native understanding of XML structure,
+    so we use XML tags for clear delineation of context types.
+
+    Features:
+    - XML tags for each context type
+    - Document structure for knowledge contexts
+    - Role-based message formatting for conversations
+    - Tool result wrapping with tool names
+    """
+
+    MODEL_PATTERNS: list[str] = ["claude", "anthropic"]
+
+    def format(
+        self,
+        contexts: list[BaseContext],
+        **kwargs: Any,
+    ) -> str:
+        """
+        Format contexts for Claude models.
+
+        Uses XML tags for structured content that Claude
+        understands natively.
+
+        Args:
+            contexts: List of contexts to format
+            **kwargs: Additional formatting options
+
+        Returns:
+            XML-structured context string
+        """
+        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 specific type for Claude.
+
+        Args:
+            contexts: List of contexts of the same type
+            context_type: The type of contexts
+            **kwargs: Additional formatting options
+
+        Returns:
+            XML-formatted string for this context type
+        """
+        if not contexts:
+            return ""
+
+        if context_type == ContextType.SYSTEM:
+            return self._format_system(contexts)
+        elif context_type == ContextType.TASK:
+            return self._format_task(contexts)
+        elif context_type == ContextType.KNOWLEDGE:
+            return self._format_knowledge(contexts)
+        elif context_type == ContextType.CONVERSATION:
+            return self._format_conversation(contexts)
+        elif context_type == ContextType.TOOL:
+            return self._format_tool(contexts)
+
+        return "\n".join(c.content for c in contexts)
+
+    def _format_system(self, contexts: list[BaseContext]) -> str:
+        """Format system contexts."""
+        content = "\n\n".join(c.content for c in contexts)
+        return f"<system_instructions>\n{content}\n</system_instructions>"
+
+    def _format_task(self, contexts: list[BaseContext]) -> str:
+        """Format task contexts."""
+        content = "\n\n".join(c.content for c in contexts)
+        return f"<current_task>\n{content}\n</current_task>"
+
+    def _format_knowledge(self, contexts: list[BaseContext]) -> str:
+        """
+        Format knowledge contexts as structured documents.
+
+        Each knowledge context becomes a document with source attribution.
+        """
+        parts = ["<reference_documents>"]
+
+        for ctx in contexts:
+            source = self._escape_xml(ctx.source)
+            content = ctx.content
+            score = ctx.metadata.get("score", ctx.metadata.get("relevance_score", ""))
+
+            if score:
+                parts.append(f'<document source="{source}" relevance="{score}">')
+            else:
+                parts.append(f'<document source="{source}">')
+
+            parts.append(content)
+            parts.append("</document>")
+
+        parts.append("</reference_documents>")
+        return "\n".join(parts)
+
+    def _format_conversation(self, contexts: list[BaseContext]) -> str:
+        """
+        Format conversation contexts as message history.
+
+        Uses role-based message tags for clear turn delineation.
+        """
+        parts = ["<conversation_history>"]
+
+        for ctx in contexts:
+            role = ctx.metadata.get("role", "user")
+            parts.append(f'<message role="{role}">')
+            parts.append(ctx.content)
+            parts.append("</message>")
+
+        parts.append("</conversation_history>")
+        return "\n".join(parts)
+
+    def _format_tool(self, contexts: list[BaseContext]) -> str:
+        """
+        Format tool contexts as tool results.
+
+        Each tool result is wrapped with the tool name.
+        """
+        parts = ["<tool_results>"]
+
+        for ctx in contexts:
+            tool_name = ctx.metadata.get("tool_name", "unknown")
+            status = ctx.metadata.get("status", "")
+
+            if status:
+                parts.append(f'<tool_result name="{tool_name}" status="{status}">')
+            else:
+                parts.append(f'<tool_result name="{tool_name}">')
+
+            parts.append(ctx.content)
+            parts.append("</tool_result>")
+
+        parts.append("</tool_results>")
+        return "\n".join(parts)
+
+    @staticmethod
+    def _escape_xml(text: str) -> str:
+        """Escape XML special characters in attribute values."""
+        return (
+            text.replace("&", "&amp;")
+            .replace("<", "&lt;")
+            .replace(">", "&gt;")
+            .replace('"', "&quot;")
+            .replace("'", "&apos;")
+        )