fix(talk): address PR #741 reviewer feedback

Four targeted fixes from the AI code review:

1. TalkConversation.description: drop the misleading `str | None`
   union (spreed always sends `""`, never null) — type is now `str`
   with default `""`.

2. get_messages: guard the X-Chat-Last-Given int parse with
   try/except so a misbehaving proxy can't crash the read flow;
   logs a warning and falls back to None.

3. get_messages: clamp `limit` to [1, 200] in the client (spreed
   caps server-side at 200 and silently truncates) so the returned
   `count` always matches what was actually requested. Both client
   and server-tool docstrings updated to state the valid range.

4. Add an integration test covering the 32000-char message ceiling
   in talk_send_message — the empty-message case was already tested,
   the over-length case was not.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/server/test_talk_mcp.py

@@ -141,3 +141,18 @@ async def test_talk_send_message_validation_empty_text(
         "talk_send_message", {"token": token, "message": ""}
     )
     assert result.isError is True, "Expected validation error for empty message"
+
+
+async def test_talk_send_message_validation_too_long(
+    nc_mcp_client: ClientSession, temporary_conversation: dict
+):
+    """A message exceeding the 32000-char ceiling is rejected client-side."""
+    token = temporary_conversation["token"]
+
+    result = await nc_mcp_client.call_tool(
+        "talk_send_message",
+        {"token": token, "message": "x" * 32001},
+    )
+    assert result.isError is True, (
+        "Expected validation error for message longer than 32000 characters"
+    )