G7: F09 — bump DEFAULT_TIMEOUT from 5s to 15s

Why: 5 seconds is too short for slow-path mesh operations (path-resolving
messaging, long binary responses, remote auth). Also the root cause of tests
that appeared to "hang" — they were falling through to the 5s timeout because
their mock dispatchers don't wire matching responses. Landed as a separate
commit so reviewers can drop it independently if they push back.

Refs: Forensics report finding F09

src/meshcore/commands/base.py

@@ -58,7 +58,7 @@ def _validate_destination(dst: DestinationType, prefix_length: int = 6) -> bytes
 
 
 class CommandHandlerBase:
-    DEFAULT_TIMEOUT = 5.0
+    DEFAULT_TIMEOUT = 15.0
 
     def __init__(self, default_timeout: Optional[float] = None):
         self._sender_func: Optional[Callable[[bytes], Coroutine[Any, Any, None]]] = None