feat(border): add dashed and dotted borderStyle for container widgets

[sharjeelyunus]

Description

Adds a borderStyle style property for container-based widgets (Row, Column, Flex*, and other BoxWrapper widgets) so borders can be solid, dashed, or dotted. This unblocks UI patterns like dashed drop zones without changing existing solid border behavior.

Related Issue

Fixes #2347

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

What Has Changed

• Added BorderStyleComposite (type, length, gap) on BoxController and HasBorderController
• Added DashedBoxBorder to paint dashed/dotted outlines (including borderRadius)
• Updated BoxWrapper / EnsembleBoxWrapper to resolve solid vs dashed/dotted vs gradient borders
• Defaults: unset/solid keeps existing behavior; length defaults to 5, gap defaults to 3

Example:

Column:
  styles:
    borderWidth: 2
    borderColor: 0xff276ef1
    borderStyle:
      type: dashed
      length: 5
      gap: 4

How to Test

1. Create a Column/Row with borderWidth, borderColor, and borderStyle.type: solid — confirm solid border (same as before).
2. Set borderStyle.type: dashed with optional length / gap — confirm dashed border.
3. Set borderStyle.type: dotted — confirm dotted border.
4. Add borderRadius with dashed/dotted — confirm corners follow the radius.
5. Omit borderStyle — confirm existing solid border behavior is unchanged.

Screenshots / Videos

Checklist

• I have run flutter analyze and addressed any new warnings
• I have run flutter test and all tests pass
• I have tested my changes on the relevant platform(s)
• I have updated documentation if needed
• My changes do not introduce new warnings or errors