fix: add input validation and tests for /forms/fill endpoint

[Mahendrareddy2006]

Summary

This PR enhances the /forms/fill endpoint by introducing proper input validation and improving overall request handling reliability. It ensures that invalid or malformed requests are rejected early, preventing unnecessary processing and potential downstream failures.

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#Changes

• Added validation to reject empty or whitespace-only 'input_text'

• Added validation to ensure 'template_id' is valid (non-null and positive)

• Introduced an optional upper limit on input length to prevent excessively large payloads

• Improved error handling using consistent 'AppError' responses with appropriate status codes (400)

• Removed redundant database calls by avoiding duplicate template fetching

• Added unit tests covering:

  • valid request flow
  • empty input handling
  • invalid template ID
  • excessively long input

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#Why

Previously, the endpoint accepted invalid inputs such as empty text or incorrect template IDs. This could lead to:

• unnecessary LLM invocations (wasting compute/resources)
• confusing error states or silent failures
• inconsistent API behavior for clients

By validating inputs at the API boundary, we ensure that only meaningful and processable requests reach the business logic layer.

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Result

• API now returns clear and consistent 400 Bad Request responses for invalid inputs
• Prevents unnecessary LLM processing and improves performance efficiency
• Improves overall API robustness and predictability
• Adds reliable test coverage to prevent regressions in the future

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Notes

• This change is backward-compatible for valid requests
• Validation logic is lightweight and does not introduce performance overhead