Slack: get_socket_mode_url creates new reqwest::Client per reconnect

[thepagent]

Parent: #382

Problem

get_socket_mode_url() creates a new reqwest::Client on every call. Since Socket Mode reconnects are rare (server-initiated disconnect, network blip), this is not a real performance issue, but it would be cleaner to reuse SlackAdapter::client.

Suggested fix

Pass &self.client or &reqwest::Client to get_socket_mode_url() instead of constructing a new one. Requires minor refactor since the function is called before SlackAdapter is constructed in the current flow.

Severity: nit — no functional impact.

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[thepagent]

Closing — low-severity Slack nit, not blocking 0.7.7 GA.