[Widgets] Implement Rust Widget APIs for Advanced Form Inputs

[rorychatt]

Problem Statement

Missing Rust backend APIs for advanced input components: AsyncSelectInput, CodeInput, ContentInput, DateRangeInput, FileInput, IconInput, NumberRangeInput.

Missing Rust Widgets

• AsyncSelectInput (async option loader)
• CodeInput (monaco code editor)
• ContentInput (rich text / markdown editor)
• DateRangeInput (start/end date picker)
• FileInput (managed upload builder)
• IconInput (icon picker)
• NumberRangeInput (min/max range slider)

Proposed Architecture & Design

1. Add inputs_advanced.rs in rusty/src/widgets/.
2. Connect with use_form validation states and async query hooks.
3. Re-export in rusty::widgets.

Acceptance Criteria

• All 7 advanced input widgets defined with full Rust builder methods.
• Validation and event binding props functional.