fix(api): return 204 No Content on failed PDF filters; add OpenAPI responses and safe resource handling

[Ludy87]

Description of Changes

• What was changed

  • Added explicit @ApiResponses for filter endpoints with 200 (PDF passed) and 204 (did not pass) including @Content and media types.
  • Replaced ambiguous null returns with ResponseEntity.noContent().build() when a filter condition is not met.
  • Ensured PDDocument is properly closed using try-with-resources in relevant endpoints.
  • Consolidated comparison logic into a reusable, type-safe compare<T extends Comparable<T>>() helper for page count, page size, file size, and rotation checks.
  • Minor cleanup and consistency improvements across filter endpoints.

• Why the change was made

  • To return correct HTTP semantics (avoid ambiguous null responses) and improve API reliability for clients consuming these endpoints.
  • To document expected responses clearly in the OpenAPI spec for better consumer tooling and DX.
  • To prevent potential resource leaks by consistently closing PDDocument.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have tested my changes locally. Refer to the Testing Guide for more details.

[Copilot]

Pull Request Overview

This PR fixes API response handling in PDF filter endpoints by implementing proper HTTP semantics. Instead of returning ambiguous null responses, the endpoints now return HTTP 204 No Content when filters fail and HTTP 200 when they pass.

• Replaced null returns with ResponseEntity.noContent().build() for failed filter conditions
• Added comprehensive OpenAPI documentation with @ApiResponses annotations
• Implemented proper resource management with try-with-resources for PDDocument instances
• Consolidated repetitive comparison logic into a reusable generic helper method

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