sdp: address review nits (import at top, InvalidPacketError)

- bumble/sdp.py: replace raise ValueError with raise InvalidPacketError
  in DataElement.list_from_bytes depth guard. InvalidPacketError
  already imported at line 34 and extends ValueError so the existing
  regression test continues to match.
- tests/sdp_test.py: remove duplicate 'import pytest' inside
  test_nested_sequence_recursion_guard; pytest already imported at
  module top (line 23).

Threading.local counter left as-is per zxzxwu's 'leave it here and
refactor later' comment on the PR.

bumble/sdp.py

@@ -294,7 +294,7 @@ class DataElement:
     def list_from_bytes(data):
         depth = getattr(_parse_state, "depth", 0)
         if depth >= _MAX_DATA_ELEMENT_NESTING:
-            raise ValueError(
+            raise InvalidPacketError(
                 f"SDP data element nesting exceeds max depth "
                 f"({_MAX_DATA_ELEMENT_NESTING})"
             )