fix(slack): add clear modal response_action for view_submission

[Franz1241]

Summary

• Adds a fifth ModalClearResponse variant ({ action: "clear" }) to the SDK's ModalResponse union, bringing the abstraction to parity with Slack's five documented view_submission response semantics.
• Wires it through the Slack adapter's modalResponseToSlack translator to emit { response_action: "clear" }, which dismisses the entire modal stack (vs. "close" / empty body, which only closes the submitted view).

Per the official Slack modals docs, view_submission supports 5 response shapes and only 4 were implemented

Test plan

pnpm validate 16/16 pass

Closes #400

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[vercel]

Additional Suggestion:

ModalClearResponse type is not exported from the chat package entry point, making it unavailable to consumers despite being part of the exported ModalResponse union type.