feat(models): Added Context class

Context stores execution state:
- variables: static test variables
- extracted: data extracted from responses
- resolve(): replaces {{var}} placeholders with values

models/context.py

@@ -0,0 +1,19 @@
+from typing import Any
+from .itm import TestModel
+
+
+class Context:
+    def __init__(self, config: TestModel):
+        self.config = config
+        self.variables: dict[str, str] = config.variables.copy()
+        self.extracted: dict[str, Any] = {}
+
+    def resolve(self, template: str) -> str:
+        result = template
+        all_vars = {**self.variables, **self.extracted}
+        for key, value in all_vars.items():
+            result = result.replace(f"{{{{{key}}}}}", str(value))
+        return result
+
+    def set_extracted(self, name: str, value: Any):
+        self.extracted[name] = value