Support Guzzle 8

[esetnik]

Summary

• allow guzzlehttp/guzzle ^8.0 while preserving ^7.8
• require no generated-client changes after exercising the major HTTP request/response paths

Closes #33.

Validation

Validated on PHP 8.5.9 with Composer 2.10.2.

Guzzle 8

composer install --no-interaction --no-progress --prefer-dist
composer validate --strict --no-check-version
composer check-platform-reqs --no-dev
composer dump-autoload --optimize --strict-psr --no-interaction
composer audit --locked --no-interaction

• a lockless install selected guzzlehttp/guzzle 8.0.1, guzzlehttp/promises 3.0.0, and guzzlehttp/psr7 3.0.0
• 156 tracked PHP files passed syntax checks
• platform, optimized PSR-4 autoload, and advisory checks passed
• a concrete GuzzleHttp\Client + MockHandler smoke covered synchronous and asynchronous calls, 4xx exception translation, response deserialization, JSON model bodies, Basic auth, query construction, and multipart SplFileObject upload

Guzzle 7

composer update \
  --with 'guzzlehttp/guzzle:^7.8' \
  --with-all-dependencies --no-interaction

• resolved guzzlehttp/guzzle 7.15.2
• the same comprehensive generated-client smoke passed

This repository does not currently contain an automated test suite, static-analysis dependency, Composer test script, or CI workflow. Strict Composer validation also retains the pre-existing warning for the fixed version field; validation passes when that unrelated check is omitted.