feat: add --file-reporter support to test commands

[unicoderbot]

Summary

Adds support for the --file-reporter option to both very_good test (Flutter) and very_good dart test commands. This option enables writing test results to a file in various formats (e.g., JSON) without affecting the CLI's stdout output.

Changes

• Added --file-reporter option to Flutter test command that accepts format in <name>:<path> form (e.g., json:reports/tests.json)
• Added --file-reporter option to Dart test command with the same format
• Updated both FlutterTestOptions and DartTestOptions classes to parse and store the file reporter value
• Updated argument passing to flutterTest() and dartTest() to include the --file-reporter flag when provided
• Added tests to verify the option is correctly passed through
• Updated documentation in site/docs/commands/test.md

Motivation

The file reporter output can be converted to popular formats like JUnit XML reports using tools like junitreport, enabling better integration with CI/CD platforms and IDEs.

Closes #331

[marcossevilla]

@vgvbot /address-review

[unicoderbot]

On it, I'll address the review comments and push the changes.

• Preparing environment
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  • Setting up dev environment
• Reviewing feedback
• Working on changes
• Pushing updates

I've pushed changes to address the feedback.