Add unit tests for ErrorMessageExtractor

[noruzzamans]

Description

This PR introduces a comprehensive PHPUnit unit test suite for the ErrorMessageExtractor utility class (WordPress\AiClient\Providers\Http\Util\ErrorMessageExtractor).

ErrorMessageExtractor centralizes the parsing and extraction of human-readable error messages from various API response shapes returned by generative AI providers (such as OpenAI, Google Gemini, Anthropic) when HTTP errors occur (ClientException and ServerException).

While the sibling ResponseUtilTest exercises high-level HTTP status code handling, ErrorMessageExtractor itself previously lacked a dedicated unit test suite. This contribution fills that gap by verifying all supported response structures, format precedence, and edge-case payload validation.

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Test Coverage Summary

The newly added test suite (tests/unit/Providers/Http/Util/ErrorMessageExtractorTest.php) contains 40 tests and 42 assertions covering:

1. Non-Array Input Handling: Asserts that extractFromResponseData safely returns null when input is not an array (null, empty string, non-empty string, integer, float, boolean, object).
2. Array-Wrapped Error Objects: Verifies extraction from [ [ 'error' => [ 'message' => '...' ] ] ] (Google Gemini / batch error format).
3. Nested Associative Error Object: Verifies extraction from [ 'error' => [ 'message' => '...' ] ] (OpenAI format).
4. Simple Error String: Verifies extraction from [ 'error' => '...' ].
5. Direct Message String: Verifies extraction from [ 'message' => '...' ].
6. Format Precedence: Confirms resolution hierarchy when multiple candidate error formats are present in the same payload.
7. Malformed & Unrecognized Data Structures: Asserts null is safely returned for empty arrays, missing keys, non-string messages (integers, booleans, arrays, nulls), and non-array array elements.

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Verification & Testing

All local test suites and linters pass cleanly:

# Isolated unit test
vendor/bin/phpunit tests/unit/Providers/Http/Util/ErrorMessageExtractorTest.php

# Full unit test suite (1242 tests, 4408 assertions)
composer test:unit

# Coding standards (PER / PSR-12)
composer phpcs

# Static analysis
composer phpstan

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Use of AI Tools

AI assistance: Yes
Model(s): Gemini 3.8 Flash High
Used for: Authoring PHPUnit unit tests, data providers, and test documentation for ErrorMessageExtractor. Code reviewed and locally validated by human contributor.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.03%. Comparing base (77a5995) to head (b5f0ee1).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #297      +/-   ##
============================================
+ Coverage     86.58%   87.03%   +0.44%     
  Complexity     1383     1383              
============================================
  Files            69       69              
  Lines          4449     4449              
============================================
+ Hits           3852     3872      +20     
+ Misses          597      577      -20     

Flag	Coverage Δ
unit	87.03% <ø> (+0.44%)	⬆️

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Co-authored-by: noruzzamans <[email protected]>

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