sdp: bound DataElement parse recursion to prevent RecursionError DoS

DataElement.from_bytes -> list_from_bytes -> (SEQUENCE/ALTERNATIVE
constructor dispatches back to list_from_bytes) had no depth limit. A
malicious SDP peer could send a PDU of a few kilobytes containing ~1000
nested SEQUENCE tags and exhaust the Python recursion stack, crashing the
host with an unhandled RecursionError propagating out of the SDP handler.

Reachable via: any remote Bluetooth device that Bumble performs SDP
service discovery against (default during Classic connection setup).

Same family as PR #912 (ATT_PDU.from_bytes empty PDU IndexError) - remote
unchecked-input parser crash in the Bluetooth stack.

Fix: thread-local depth counter, cap nesting at 32 (well above anything a
legitimate service record uses). Added two regression tests covering the
deep-nesting reject path and normal 16-level-nested SEQUENCE parsing.

Reproducer (4.5 KB payload, deterministic crash on 0.0.228):

    from bumble.sdp import DataElement
    inner = b"\x35\x00"
    for _ in range(1500):
        size = len(inner)
        if size < 65535:
            inner = bytes([0x36, (size >> 8) & 0xFF, size & 0xFF]) + inner
    DataElement.from_bytes(inner)  # RecursionError before fix

Signed-off-by: ibondarenko1 <ibondarenko1@users.noreply.github.com>

bumble/sdp.py

@@ -20,6 +20,7 @@ from __future__ import annotations
 import asyncio
 import logging
 import struct
+import threading
 from collections.abc import Iterable, Sequence
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, ClassVar, NewType, TypeVar
@@ -44,6 +45,13 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
+# SDP data elements are nested (SEQUENCE, ALTERNATIVE). Cap parse recursion to
+# prevent a malicious peer from crashing the process via a deeply nested PDU.
+# 32 levels is well beyond anything a legitimate service record uses.
+_MAX_DATA_ELEMENT_NESTING = 32
+_parse_state = threading.local()
+
+
 # -----------------------------------------------------------------------------
 # Constants
 # -----------------------------------------------------------------------------
@@ -284,12 +292,22 @@ class DataElement:
 
     @staticmethod
     def list_from_bytes(data):
-        elements = []
-        while data:
-            element = DataElement.from_bytes(data)
-            elements.append(element)
-            data = data[len(bytes(element)) :]
-        return elements
+        depth = getattr(_parse_state, "depth", 0)
+        if depth >= _MAX_DATA_ELEMENT_NESTING:
+            raise ValueError(
+                f"SDP data element nesting exceeds max depth "
+                f"({_MAX_DATA_ELEMENT_NESTING})"
+            )
+        _parse_state.depth = depth + 1
+        try:
+            elements = []
+            while data:
+                element = DataElement.from_bytes(data)
+                elements.append(element)
+                data = data[len(bytes(element)) :]
+            return elements
+        finally:
+            _parse_state.depth = depth
 
     @staticmethod
     def parse_from_bytes(data, offset):