Add validation for input_text in fill_form endpoint

[Lochit-Vinay]

Closes #376

Summary

Added validation for input_text in the fill_form endpoint to prevent empty or whitespace-only input.

Problem

The endpoint allowed empty or whitespace-only input_text, which could lead to:

• Unnecessary processing
• Unexpected behavior in downstream components

Before

fetched_template = get_template(db, form.template_id)

Solution

Added input validation at the beginning of the function:

After

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

fetched_template = get_template(db, form.template_id)

Benefits

• Prevents invalid requests
• Improves API robustness
• Reduces unnecessary processing
• Ensures early validation, preventing unnecessary downstream execution