simplex-chat-python: split Client from Bot, add request/response API (#6976)

* simplex-chat-python: split Client from Bot, add request/response API

Client is now the base class for SimpleX participants that talk TO
services (monitors, probes, automated participants). Bot extends Client
with server features (address, auto-accept, welcome, commands).

New methods on Client (inherited by Bot):
  connect_to(link)           idempotent contact handshake
  send_and_wait(id, text)    send a message and await the reply
  events()                   async iterator over chat events
  @on_message(contact_id=N)  filter by sender in decorators

BotProfile renamed to Profile (alias kept). New ContactAlreadyExistsError
subclass for cleaner error handling.

* simplex-chat-python: narrow event payload type per @on_event tag

@client.on_event("contactConnected") now types the handler's event
parameter as CEvt.ContactConnected instead of the unnarrowed
CEvt.ChatEvent union — mirroring how @on_message narrows by
content_type.

The 50 overloads are generated by the Haskell codegen into _events.py
(as a Protocol class), so new events stay in sync automatically.
Client.on_event is exposed as a property typed as that Protocol; the
runtime implementation is unchanged.

packages/simplex-chat-python/src/simplex_chat/api.py

@@ -40,10 +40,26 @@ def _db_to_migrate_args(db: Db) -> tuple[str, str, _native.Backend]:
 
 
 class ChatCommandError(Exception):
+    """A chat command returned an unexpected response type.
+
+    `response` is the raw wire response; `response_type` exposes its `type`
+    discriminator for quick checks. Subclasses cover known recoverable cases
+    so callers can `except ContactAlreadyExistsError` instead of inspecting
+    `response.get("type")` themselves.
+    """
+
     def __init__(self, message: str, response: CR.ChatResponse):
         super().__init__(message)
         self.response = response
 
+    @property
+    def response_type(self) -> str:
+        return self.response.get("type", "")  # type: ignore[return-value]
+
+
+class ContactAlreadyExistsError(ChatCommandError):
+    """`api_connect`/`api_connect_active_user` was called for an existing contact."""
+
 
 class ChatApi:
     def __init__(self, ctrl: int):
@@ -481,7 +497,7 @@ class ChatApi:
         if r["type"] == "sentInvitation":
             return "contact"
         if r["type"] == "contactAlreadyExists":
-            raise ChatCommandError("contact already exists", r)
+            raise ContactAlreadyExistsError("contact already exists", r)
         raise ChatCommandError("connection error", r)
 
     async def api_accept_contact_request(self, contact_req_id: int) -> T.Contact: