Upgrade saloonphp/saloon from v3 to v4

[pushpak1300]

Saloon v4 was released as a security update resolving three CVEs: insecure deserialization (RCE), absolute URL endpoint override (SSRF), and fixture path traversal. This upgrades to v4 to eliminate those vulnerabilities.

Approach

• Bumped saloonphp/saloon to ^4.0, saloonphp/cache-plugin to ^3.1, and saloonphp/pagination-plugin to ^2.3
• No code changes required — the three breaking changes (AccessTokenAuthenticator serialization removal, opt-in base URL override, fixture path traversal restriction) don't affect this project