Remove 1000 character limit from textfield

[pamelafox]

Purpose

Fixes #2399

Multiple developers have wanted to accept inputs greater than 1000 characters.
We discussed with Pablo and Matt here:
#952

Pablo said:
"I suspect this was originally in place to make sure the input + instructions didn't exceed the context window limit, but those limits have gone up, and there's probably better ways to restrict this if needed. As a general good practice, you still want a limit (either a higher one or a more elaborate one that depends on the target LLM's max prompt length) since you should not allow inputs that haven't been tested, so you need a test that tries whatever static/dynamic max length input you want to allow."

This PR removes the limit entirely, making it possible for the developer to paste massive text and exceed the content length of the model. They should see an appropriate error in that case, however. I can verify that/ possibly add a unit test.

Does this introduce a breaking change?

When developers merge from main and run the server, azd up, or azd deploy, will this produce an error?
If you're not sure, try it out on an old environment.

[ ] Yes
[X] No

Does this require changes to learn.microsoft.com docs?

This repository is referenced by this tutorial
which includes deployment, settings and usage instructions. If text or screenshot need to change in the tutorial,
check the box below and notify the tutorial author. A Microsoft employee can do this for you if you're an external contributor.

[ ] Yes
[X] No

Type of change

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[X] Other... Please describe:

Code quality checklist

See CONTRIBUTING.md for more details.

• The current tests all pass (python -m pytest).
• I added tests that prove my fix is effective or that my feature works
• I ran python -m pytest --cov to verify 100% coverage of added lines
• I ran python -m mypy to check for type errors
• I either used the pre-commit hooks or ran ruff and black manually on my code.