Validate input_text in fill_form endpoint

[Lochit-Vinay]

Problem

The fill_form endpoint currently does not validate the input_text field.

Empty or whitespace-only input can be passed, which may lead to:

• Unnecessary processing
• Unexpected behavior in downstream components

Proposed Solution

Add validation to ensure that input_text is not empty or whitespace-only.

Example:

if not form.input_text.strip():
    raise AppError("Input text cannot be empty", status_code=400)

I would like to work on this and submit a PR.