Properly handle list of approvers even if user doesn't exists anymore in Slack

[IvanVojtko]

This is not properly handled for group request access. It's correctly handled if we use find_approvers_in_slack instead of directly getting list from get_user_by_email which throws an exception when user is not found. This PR fixes #138

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Note

Improves resilience of approver resolution for group access requests.

• Replaces direct get_user_by_email usage with slack_helpers.find_approvers_in_slack in src/group.py under DecisionReason.RequiresApproval to avoid exceptions when users no longer exist in Slack
• Adds explicit handling when no approvers are found: send error text/DM and set bad_result_emoji
• If some approver emails are missing, appends a note listing the missing emails while proceeding with mentions of found approvers and waiting_result_emoji
• Adjusts channel and DM texts accordingly while preserving existing flow for other decision paths

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[Andrey9kin]

@IvanVojtko thanks for the PR, will look into it tomorrow