fix: remove double-serialisation of payload in component-client.js

Closes #11 — payload was JSON.stringify'd before embedding in the request
body, then the body itself was stringified again. Server received payload
as a string instead of an object, causing TypeError on handler(**payload).

- Removed inner JSON.stringify(payload) in component-client.js
- Added server-side guards in FastAPI and Litestar adapters to handle
  double-serialised payloads from cached older JS (Django already had this)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: fsecada01

src/component_framework/adapters/fastapi.py

@@ -1,5 +1,6 @@
 """FastAPI adapter for component endpoints."""
 
+import json
 import logging
 
 try:
@@ -47,9 +48,18 @@ async def component_endpoint(name: str, request: Request) -> JSONResponse:
         # Extract parameters
         params = data.get("params", {})
         event = data.get("event")
-        payload = data.get("payload", {})
+        payload_raw = data.get("payload", {})
         state_str = data.get("state")
 
+        # Guard against double-serialised payload from older client JS
+        if isinstance(payload_raw, str):
+            try:
+                payload = json.loads(payload_raw)
+            except (json.JSONDecodeError, ValueError):
+                payload = {}
+        else:
+            payload = payload_raw
+
         # Deserialize state if provided
         state = None
         if state_str:

src/component_framework/adapters/litestar.py

@@ -1,5 +1,6 @@
 """Litestar adapter for component endpoints."""
 
+import json
 import logging
 
 try:
@@ -49,9 +50,18 @@ async def component_endpoint(name: str, request: Request) -> Response:
         # Extract parameters
         params = data.get("params", {})
         event = data.get("event")
-        payload = data.get("payload", {})
+        payload_raw = data.get("payload", {})
         state_str = data.get("state")
 
+        # Guard against double-serialised payload from older client JS
+        if isinstance(payload_raw, str):
+            try:
+                payload = json.loads(payload_raw)
+            except (json.JSONDecodeError, ValueError):
+                payload = {}
+        else:
+            payload = payload_raw
+
         # Deserialize state if provided
         state = None
         if state_str:

src/component_framework/static/component_framework/js/component-client.js

@@ -152,7 +152,7 @@ class ComponentClient {
 
     const body = {
       event,
-      payload: JSON.stringify(payload),
+      payload,
       ...(stateJson ? { state: stateJson } : {}),
     };