Refactor: move input_text validation to schema level using Pydantic validators

[Lochit-Vinay]

Closes #378

Summary

Moved input validation for input_text from the endpoint to the schema level using Pydantic validators.

Problem

Validation was previously handled inside the route, leading to scattered and non-reusable logic.

Before

Validation was implemented inside the endpoint:

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

After

Validation is now centralized in the schema:

@field_validator("input_text")
def validate_input_text(cls, value):
    if not value.strip():
        raise ValueError("Input text cannot be empty")
    return value

Solution

Added a field validator in the FormFill schema and removed manual validation from the route.

Benefits

• Cleaner endpoint code
• Centralized validation
• Improved maintainability
• Aligns with FastAPI best practices