feat(slack): support modal selection change events

[dancer]

summary

• add optional dispatchAction to Select and RadioSelect builders and JSX props
• enable Slack modal selection changes through existing onAction handlers, allowing dependent fields to update before submission
• preserve default-off behavior and existing submission values
• document Slack-specific behavior and view update guidance

test plan

• verify builder, React and JSX option propagation
• verify Slack input blocks with enabled, disabled and omitted options
• test signed modal actions, dependent field updates, cleared selections and final submissions through the local Slack emulator
• full build, typecheck and test suite pass
• tracked-file lint and affected-package Knip pass
• full pnpm validate is blocked by unrelated ignored local projects
• checked official Slack documentation; live Slack UI testing not performed

Fixes #926

[vercel]

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Project	Deployment	Actions	Updated
chat	Ready	Preview, v0	Sep 17, 2026 6:16pm UTC
chat-sdk-nextjs-chat	Ready	Preview, v0	Sep 17, 2026 6:16pm UTC