refactor(adapters/slack): move remaining adapter calls onto typed slackapi methods #100

Follow-up from #97. The Slack adapter now uses slackapi for transport, retry, signature checks, and history reads. It still builds its own request bodies for four methods:

• auth.test
• chat.postMessage
• chat.postEphemeral
• conversations.open

Move these onto the typed slackapi methods so the adapter has one request encoding per method. The existing adapter hardening tests are the acceptance bar: they must keep passing without being weakened.

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