Feat: support variable interpolation in headers (#13680)

Closes #13277

### What problem does this PR solve?

Adds `{variable_name}` (and `{component@variable}`) interpolation
support to HTTP header values in the `Invoke` component, matching the
existing URL interpolation behavior.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

<img width="1280" height="867" alt="image"
src="https://github.com/user-attachments/assets/8ab7b4e9-7cc0-4a7f-8a5f-f838a15a5fda"
/>

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

agent/component/invoke.py

@@ -97,8 +97,10 @@ class Invoke(ComponentBase, ABC):
             except Exception:
                 return ""
 
+        variable_pattern = r"\{([a-zA-Z_][a-zA-Z0-9_.@-]*)\}"
+
         # {base_url} or {component_id@variable_name}
-        url = re.sub(r"\{([a-zA-Z_][a-zA-Z0-9_.@-]*)\}", replace_variable, url)
+        url = re.sub(variable_pattern, replace_variable, url)
 
         if url.find("http") != 0:
             url = "http://" + url
@@ -106,7 +108,25 @@ class Invoke(ComponentBase, ABC):
         method = self._param.method.lower()
         headers = {}
         if self._param.headers:
-            headers = json.loads(self._param.headers)
+            try:
+                parsed_headers = json.loads(self._param.headers)
+            except json.JSONDecodeError as e:
+                logging.warning(
+                    "Invoke headers are not valid JSON, ignoring headers. raw=%r error=%s",
+                    self._param.headers,
+                    e,
+                )
+                parsed_headers = {}
+            if not isinstance(parsed_headers, dict):
+                logging.warning(
+                    "Invoke headers JSON is of type %s, expected an object; ignoring headers.",
+                    type(parsed_headers).__name__,
+                )
+                parsed_headers = {}
+            headers = parsed_headers
+            for key, value in list(headers.items()):
+                if isinstance(value, str):
+                    headers[key] = re.sub(variable_pattern, replace_variable, value)
         proxies = None
         if re.sub(r"https?:?/?/?", "", self._param.proxy):
             proxies = {"http": self._param.proxy, "https": self._param.proxy}