Add metadata-driven inputs for Entity actions

[thsnkhn]

Problem

Entity actions accept raw JSON input, but their public definition has no input schema. Studio can invoke a selection action only with Record IDs. Apps must build one-off UI or parse undocumented payloads for actions that need a date, note, option, Link, number, or confirmation.

Required behavior

• Extend the public Entity action definition with metadata-driven input fields.
• Reuse Entity field behavior where it fits. Do not create a second field type system.
• Support required values, defaults, options, Link inputs, validation, and confirmation text.
• Validate the payload on the server before the handler runs.
• Render a standard Studio dialog from the action definition.
• Load Link choices through permission-aware Record queries.
• Return field-level validation details through the existing action error envelope.

Acceptance criteria

• An App can define an action with typed inputs through the public SDK.
• Metadata exposes the input contract to Studio.
• Studio opens a standard input dialog and sends a validated payload.
• Record, selection, and collection actions use the same input contract.
• Action handlers do not need to import framework internals.