fix(socket_mode): shut down current_session_runner in close()

[animaartificialis]

Summary

SocketModeClient starts three IntervalRunner threads in __init__: current_session_runner (0.1 s loop), current_app_monitor (5 s) and message_processor (0.001 s). close() shut down two of them but left current_session_runner alive, so every instance leaked one 100 ms-loop thread. In long-running watchers that recreate the client (e.g. on transient disconnects detected via is_connected()) those threads accumulate and drive CPU usage up.

This PR adds a guarded current_session_runner.shutdown() call alongside the existing two, plus a regression test verifying that all three runners exit after close().

Closes #1873

Verification

import threading, time
from slack_sdk.socket_mode.builtin.client import SocketModeClient

baseline = threading.active_count()
for _ in range(10):
    SocketModeClient(app_token="xapp-fake").close()
time.sleep(0.3)
# Before this patch: delta = 10. After: delta = 0.
print("delta =", threading.active_count() - baseline)

The added unit test test_close_shuts_down_all_runners covers this in the existing tests/slack_sdk/socket_mode/test_builtin.py.

Requirements

• Tests added
• CLA — will be signed at cla.salesforce.com after this PR is opened

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @animaartificialis to sign the Salesforce Inc. Contributor License Agreement.

[hello-ashleyintech]

hi @animaartificialis! Thank you so much for your submission. Could you make sure you sign the CLA?