Add PHPUnit tests for hook, WP, and bidirectional functions

Adds 109 PHPUnit tests covering three function files from includes/:

- acf-hook-functions.php (28 tests): Filter/action variations registration
  and application, deprecated hook handling and backward compatibility
- acf-wp-functions.php (53 tests): Object type resolution, post ID decoding,
  REST base resolution, and object ID extraction from various WordPress types
- acf-bidirectional-functions.php (28 tests): Bidirectional relationship
  target types, choice building, value updates, and field settings rendering

All tests use WorDBless for lightweight WordPress environment simulation and
include data providers for comprehensive edge case coverage.

Part of Phase 1 testing strategy implementation.

Author: priethor