Add character limit to legal form details field

[anandnikhil45]

This PR adds a character limit to the legal form details field to prevent excessively large submissions and reduce potential abuse.

Fixes #10105

[stevejalim]

Hi @anandnikhil45 - thanks for this. In the comments on the issue I think Daniel suggested 5000 chars as a good balance between too long and too short. Could you update your changes to match, please?

It would also be nice to add help text on the field to mention this limit, if you don't mind.

Thanks
Steve

[anandnikhil45]

I’ve updated the input_details field to use a 5000 character limit and added help text to document the limit, following the suggestion in the comments.

Please let me know if any further changes are needed. Thanks!

[stevejalim]

@anandnikhil45 Thanks for this. Looking at how we do help text for https://www.mozilla.org/en-US/about/legal/defend-mozilla-trademarks/, I can see we actually put it in the HTML not on the form. Would you mind making this change to make it consistent?

Here's the current markup for that page and the help text would go in on line 156 I think: https://github.com/mozilla/bedrock/blob/main/bedrock/legal/templates/legal/fraud-report.html#L119-L183

[stevejalim]

@anandnikhil45 To make the pre-commit and linting checks work, our docs mention how to install pre-commit here https://mozmeao.github.io/platform-docs/getting-started/install/?h=pre+commit#pre-commit-hooks

Once installed you can automatically fix up most linting issues with ruff check . --fix