Add PSR-12 enforcement via PHP_CodeSniffer

[muhammadshaharyaraulakh]

📋 Description

Brief description of the changes made in this PR.

Enforces PSR-12 coding style across the src/ directory by integrating PHP_CodeSniffer. Auto-fixed existing violations using PHPCBF and manually resolved remaining warnings to achieve a fully clean baseline — zero errors, zero warnings.
✨ Type of Change

🐛 Bug fix (fixes an issue)
✨ New feature (adds functionality)
🔧 Refactoring (code restructuring without functional changes)
📚 Documentation (updates to documentation)
🧪 Tests (adding or modifying tests)
🔒 Security (security-related fixes)
⚡ Performance (performance improvements)
🎨 Style (formatting changes, no logic changes)

🔍 Main Changes

Added squizlabs/php_codesniffer to require-dev in composer.json
Added .phpcs.xml configuration file targeting PSR-12 for the src/ directory
Added composer run cs and composer run cs-fix scripts via phpcs and phpcbf
Auto-fixed formatting violations in src/OauthClient.php and src/Transports/CurlTransport.php using PHPCBF
Manually fixed constant visibility warnings in src/OauthClient.php
Manually fixed long method signature in src/ApiException.php
Suppressed intentional PSR-1 side-effects warning in src/Bootstrap.php

🧪 Testing

I have tested the changes locally
I have added/updated unit tests
All tests pass (./vendor/bin/phpunit)
I have verified PHP 8.0+ compatibility

📝 Additional Notes
vendor/bin/phpcs reports zero errors and zero warnings across all 10 scanned files. No logic or functionality was changed — only formatting and style enforcement tooling was added.
🔗 Related Issue
Closes #22

[muhammadshaharyaraulakh]

Ready for review @francescobianco vendor/bin/phpcs reports zero errors and zero warnings locally. Happy to make any adjustments based on your feedback