fix: Remove unused RunContext import causing runtime error

RunContext was imported but never used at runtime in get_agent().
Also updated TYPE_CHECKING import to use CallContext alias for compatibility.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

Author: OxFrancesco

imagine/agent.py

@@ -7,7 +7,7 @@
 from imagine.services.r2 import R2Service
 
 if TYPE_CHECKING:
-    from pydantic_ai import RunContext
+    from pydantic_ai import CallContext as RunContext
 
 # Lazy agent initialization to avoid import errors when OPENROUTER_API_KEY is not set
 _agent: Any = None
@@ -156,7 +156,7 @@ def get_agent() -> Any:
     """
     global _agent
     if _agent is None:
-        from pydantic_ai import Agent, RunContext
+        from pydantic_ai import Agent
 
         _agent = Agent(
             'openrouter:anthropic/claude-haiku-4.5',