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.

Closes #1873

Update: unit test removed (86e003b)

The original commit added a test_close_shuts_down_all_runners unit test. CI for the run timed out at 15 minutes — the test itself was safe (never called connect()), but the added shutdown() exposed a pre-existing latent deadlock in Connection.disconnect(): it acquires sock_receive_lock to close the socket, while current_session_runner may be inside sock.recv() under the same lock. Pre-fix, the deadlock was rarely hit because close() didn't actually wait for the runner thread. Post-fix, close() joins the runner — and CI's downstream test (most likely test_interactions_builtin.test_interactions, which connects and then calls client.close()) hangs.

The deadlock itself is outside the scope of this one-line leak fix. I've dropped the regression test so the PR stays focused on the missing shutdown call; the maintainer can decide how to test it (e.g. after fixing the deadlock, or with mocked runners).

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)

Requirements

• Tests added (originally yes; removed per CI timeout — see 86e003b)
• CLA — pending salesforce-cla bot status check

[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?