Fix Slack API calls that require form-urlencoded encoding (#7)

conversations.members, conversations.open, and users.info reject
JSON body with 'missing required field'. Use form-urlencoded for
these methods while keeping JSON for chat.postMessage and
files.upload which require it for structured blocks/payloads.

src/shared/shared/slack.py

@@ -1,7 +1,8 @@
 import json
 import os
-import urllib.request
 import urllib.error
+import urllib.parse
+import urllib.request
 from shared.secrets import get_secret, get_parameter
 
 _token = None
@@ -18,14 +19,19 @@ def _get_channel() -> str:
     return get_parameter(os.environ["SLACK_CHANNEL_PARAM"])
 
 
-def _api_call(method: str, payload: dict) -> dict:
+def _api_call(method: str, payload: dict, use_json: bool = True) -> dict:
+    if use_json:
         data = json.dumps(payload).encode()
+        content_type = "application/json; charset=utf-8"
+    else:
+        data = urllib.parse.urlencode(payload).encode()
+        content_type = "application/x-www-form-urlencoded"
     req = urllib.request.Request(
         f"https://slack.com/api/{method}",
         data=data,
         headers={
             "Authorization": f"Bearer {_get_token()}",
-            "Content-Type": "application/json",
+            "Content-Type": content_type,
         },
     )
     with urllib.request.urlopen(req) as resp:
@@ -40,7 +46,7 @@ def post_channel_message(text: str, blocks: list[dict] | None = None):
 
 
 def send_dm(user_id: str, text: str, blocks: list[dict] | None = None):
-    open_resp = _api_call("conversations.open", {"users": user_id})
+    open_resp = _api_call("conversations.open", {"users": user_id}, use_json=False)
     if not open_resp.get("ok"):
         return open_resp
     channel = open_resp["channel"]["id"]
@@ -69,7 +75,7 @@ def get_channel_members(channel_id: str) -> list[str]:
         payload = {"channel": channel_id, "limit": 200}
         if cursor:
             payload["cursor"] = cursor
-        resp = _api_call("conversations.members", payload)
+        resp = _api_call("conversations.members", payload, use_json=False)
         if not resp.get("ok"):
             break
         members.extend(resp.get("members", []))
@@ -80,7 +86,7 @@ def get_channel_members(channel_id: str) -> list[str]:
 
 
 def get_user_info(user_id: str) -> dict | None:
-    resp = _api_call("users.info", {"user": user_id})
+    resp = _api_call("users.info", {"user": user_id}, use_json=False)
     if resp.get("ok"):
         return resp["user"]
     return None