Add mainAxisSize property for flexible sizing

[ArAmM7]

Exposes the mainAxisSize property on MaterialPinField, MaterialPinFormField, and MaterialPinRow, which was previously hardcoded to MainAxisSize.min.

This allows users to set MainAxisSize.max so that mainAxisAlignment values like spaceBetween and spaceEvenly work as expected when the row should fill available width.

Defaults to MainAxisSize.min — no breaking changes.

[Copilot]

Pull request overview

This PR adds a mainAxisSize property to MaterialPinField, MaterialPinFormField, and MaterialPinRow widgets to allow control over horizontal space allocation. Previously, the row size was hardcoded to MainAxisSize.min. The new property defaults to MainAxisSize.min (preserving backward compatibility) but can be set to MainAxisSize.max to make the row expand and fill available width, enabling proper use of spacing alignments like spaceBetween and spaceEvenly.

Changes:

• Added mainAxisSize parameter with default value MainAxisSize.min to MaterialPinRow, MaterialPinField, and MaterialPinFormField
• Added comprehensive test coverage verifying default behavior, property pass-through, and layout behavior for both min and max values
• Minor formatting improvement in test file (removed unnecessary line break)

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/pin_code_fields/lib/src/material/layout/material_pin_row.dart	Base widget: replaced hardcoded MainAxisSize.min with mainAxisSize property parameter
packages/pin_code_fields/lib/src/material/material_pin_field.dart	Added mainAxisSize property and passed it through to MaterialPinRow
packages/pin_code_fields/lib/src/material/form/material_pin_form_field.dart	Added mainAxisSize property and passed it through to MaterialPinRow
packages/pin_code_fields/test/material_pin_field_test.dart	New test file with comprehensive tests for mainAxisSize behavior
packages/pin_code_fields/test/material_pin_form_field_test.dart	Added mainAxisSize test group and minor formatting fix

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