Feature: Add Fill Tool command pipeline

[DevAnuragT]

Add Fill Tool command pipeline in Flutter

Jira ticket: NA

This PR adds the Fill Tool implementation end-to-end in Paintroid Flutter, including command creation, serialization wiring, provider/toolbox integration, and focused unit tests.

New Features and Enhancements

• Added FillTool with flood-fill behavior and command generation.
• Added FillCommand to store/render fill results.
• Added fillToolProvider and connected Fill tool selection in toolbox state.
• Added createFillCommand(...) in command factory.
• Added Fill command deserialization routing in Command.fromJson(...).
• Added serializer type/version support for Fill command.

Refactorings and Bug Fixes

• Updated version strategy interfaces and test dummy strategy to include Fill command version.
• Added/updated focused unit tests for Fill command factory and Fill tool behavior.
• Kept changes scoped to Fill feature files and related serialization wiring.

Video Proof

paintroid_filltool.mp4

Checklist

Your checklist for this pull request

Please review the contributing guidelines and wiki pages of this repository.

• Include the name of the Jira ticket in the PR’s title
• Add the link to the ticket in Jira in the description of the PR
• Include a summary of the changes plus the relevant context
• Choose the proper base branch (develop)
• Confirm that the changes follow the project’s coding guidelines (Wiki)
• Verify that the changes generate no compiler or linter warnings
• Perform a self-review of the changes
• Verify to commit no other files than the intentionally changed ones
• Include reasonable and readable tests verifying the added or changed behavior
• Confirm that new and existing relevant/targeted tests pass locally
• Check that the commits’ message style matches the project’s guideline
• Verify that your changes do not have any conflicts with the base branch
• After the PR, verify that all CI checks have passed
• Add new information to the Wiki