feat: support {{variable}} in HTTP tool URL and add variable extraction timing option

- Apply render_template() to the endpoint URL at runtime so
  {{initial_context.*}} and {{gathered_context.*}} work in URL
  paths (same logic as node prompts and preset parameters).
- Update URL validation to allow template variables in the path
  but reject them in the domain.
- Add variable_extraction_timing setting (before/after/both) to
  HTTP tools, controlling when conversation variable extraction
  runs relative to the tool call.
- Add Variable Extraction Timing radio group to the tool Settings
  tab with Before/After/Both options.

api/services/workflow/pipecat_engine_custom_tools.py

@@ -378,6 +378,14 @@ class CustomToolManager:
                         )
                     )
 
+                # Determine when to run variable extraction relative to the tool call
+                extraction_timing = config.get("variable_extraction_timing", "before")
+
+                if extraction_timing in ("before", "both"):
+                    await self._engine._perform_variable_extraction_if_needed(
+                        self._engine._current_node, run_in_background=False
+                    )
+
                 result = await execute_http_tool(
                     tool=tool,
                     arguments=function_call_params.arguments,
@@ -386,6 +394,11 @@ class CustomToolManager:
                     organization_id=await self.get_organization_id(),
                 )
 
+                if extraction_timing in ("after", "both"):
+                    await self._engine._perform_variable_extraction_if_needed(
+                        self._engine._current_node, run_in_background=False
+                    )
+
                 await function_call_params.result_callback(result)
 
             except Exception as e: