fix(protocol): actually bump submodule pointer to v1.46.3

The previous commit (3c7bae2) hand-regenerated the .py/.pyi stubs but
never staged the submodule pointer bump itself. The build-protocol.yml
bot then re-ran generate_proto.sh against the still-recorded 0bb4e41
submodule and overwrote my CustomEvent stubs (because they didn't
match the actually-committed submodule state). Bot was right; my
commit was internally inconsistent.

Bump the submodule pointer for real (0bb4e41 -> b89c3a7, v1.46.3) so
the bot's next regen produces stubs that actually include
AgentSessionEvent.custom_event + CustomEvent.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: toubatbrian