fix: Dropdown text not clearing when its value is reset

[ndonkoHenri]

Fix #5119

Example

import flet as ft


def main(page: ft.Page):
    fruits = [
        "Apple",
        "Banana",
        "Cherry",
    ]

    def handle_dropdown_clear(e):
        d1.value = None
        d2.value = None

    options = [ft.DropdownOption(f) for f in fruits]

    page.add(
        d1 := ft.Dropdown(
            label="Dropdown",
            options=options,
        ),
        d2 := ft.DropdownM2(
            label="DropdownM2",
            options=options,
        ),
        ft.Button("Clear", on_click=handle_dropdown_clear),
    )


ft.run(main)

Summary by Sourcery

Fix dropdown text not clearing on value reset, improve state synchronization and refactor item-building logic

Bug Fixes:

• Ensure dropdown text is cleared when its value is programmatically reset

Enhancements:

• Track internal value and suppress text-change events when updating the controller
• Refactor dropdown items construction using collection utilities with null-safe filtering
• Simplify Dart code style with var declarations and remove obsolete commented code

Documentation:

• Update Python SDK docstrings for Dropdown and DropdownOption with clearer formatting and examples

[sourcery-ai]

We've reviewed this pull request using the Sourcery rules engine

[Copilot]

Pull Request Overview

This PR improves the Dropdown control implementation with better synchronization between the text controller and value changes, along with documentation improvements.

• Enhanced text controller synchronization to prevent unwanted text change events during programmatic updates
• Improved null safety handling for dropdown options with explicit validation
• Updated Flutter API usage from deprecated withOpacity to withValues

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
sdk/python/packages/flet/src/flet/controls/material/dropdown.py	Reformatted docstrings for better clarity and consistency, fixed example to use ft.DropdownOption instead of ft.dropdown.Option
packages/flet/lib/src/controls/dropdown.dart	Added state tracking for value changes, implemented text suppression mechanism, improved option validation with null handling, and updated deprecated color API

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[FeodorFitsner]

Notes:

1. Remove Example admonition.
2. Check copilot notes.