[OMEGA-429] Update channel mock suites for the V2 mock

[TossSky]

The Slack, Telegram and WebSocket mock suites on the #349 branch still passed mock answers as V1 strings, which the V2 mock turns into an empty response, so their tests timed out.

• channel tests pass answers as (tool, {param: value}) lists
• LlmMockAgent.chat strips the Step <time>: prefix before matching, so prompts with a sender prefix match again
• fixed the syntax error in test_skill_query_slack_mock.py and the import in test_slack_unwrap.py

Tested on an image built from this branch:

• @run_mandatory: 153 passed
• @run_optional: 5 passed, 1 skipped
• mock_websocket/: 13 passed
• mock_slack/: 32 passed, 1 skipped, 1 failed on a git clone timeout that passed on rerun
• mock_telegram/: 23 passed, 1 skipped

[vsbogd]

@TossSky thanks for fixing it, actually I am not sure to which branch this PR should be raised. Maybe it is better to raise it into #358 it depends on which PR we are going to merge first probably. It depends on which ticket will go to the QA state first. Let this PR hang for some time.

[TossSky]

@vsbogd
Agreed, let's leave it open until it's clear which PR goes first. I tested #349 and posted the results in a comment there.