chore(mcp): shorten the changeset and guard the persistence wording

Matches the TypeScript PR: the changeset is one user-facing line, and
the schema unit test asserts the cross-message clause itself, so removing
it from the description fails the test.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: gesh

.sampo/changesets/persistent-conversation-id.md

@@ -2,4 +2,4 @@
 pypi/posthog: patch
 ---
 
-Tell agents to keep passing the same `conversation_id` across later user messages. The previous wording let ChatGPT treat the handle as scoped to one message's tool calls, so each new message started a new session.
+Keep MCP conversation sessions consistent across user messages.

posthog/test/mcp/test_units.py

@@ -230,6 +230,11 @@ def test_add_conversation_id_adds_property():
         {"type": "object", "properties": {"x": {"type": "string"}}}, "t"
     )
     assert out["properties"]["conversation_id"]["type"] == "string"
+    # Without this clause, ChatGPT drops the handle on each new user message and every message becomes its own session.
+    assert (
+        "rest of the conversation, including after later user messages"
+        in out["properties"]["conversation_id"]["description"]
+    )
 
 
 def test_add_conversation_id_skips_when_already_present():