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.

tests/sdp_test.py

@@ -461,8 +461,6 @@ def test_nested_sequence_recursion_guard():
             break
         inner = bytes([0x36, (size >> 8) & 0xFF, size & 0xFF]) + inner
 
-    import pytest
-
     with pytest.raises(ValueError, match="nesting exceeds max depth"):
         DataElement.from_bytes(inner)