fix: thermald crash blocking shutdown (getBool → get_bool)

Python Params uses snake_case; the C++ camelCase call raised
AttributeError, killing thermald_thread at the exact moment of shutdown.
Result: DoShutdown never got set, the 10-minute timer "worked" once (set
DashcamShutdown=True) and then thermald died silently. Device kept
draining the battery instead of powering down.

Caught because CLAUDE.md specifically flags this pattern as a common
source of silent failures between C++ and Python.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

selfdrive/thermald/thermald.py

@@ -425,7 +425,7 @@ def thermald_thread(end_event, hw_queue) -> None:
       params.put_bool("DashcamShutdown", True)
       deadline = time.monotonic() + 15.0
       while time.monotonic() < deadline:
-        if not params.getBool("DashcamShutdown"):
+        if not params.get_bool("DashcamShutdown"):
           cloudlog.info("dashcamd shutdown ack received")
           break
         time.sleep(0.5)