[Schema] Fix PHPDoc type of GetPromptRequest::$arguments

[vjik]

Fix PHPDoc type of GetPromptRequest::$arguments from array<string, mixed> to array<string, string>

Motivation and Context

The MCP specification defines prompt arguments as a map of string keys to string values. The PHPDoc annotation for GetPromptRequest::$arguments incorrectly used array<string, mixed>, which was overly permissive and inconsistent with the spec. This fix aligns the type hint with the actual contract.

How Has This Been Tested?

The change is documentation-only (PHPDoc annotation) and does not affect runtime behavior. Existing tests continue to pass.

Breaking Changes

None. This is a PHPDoc-only change with no impact on runtime behavior or public API signatures.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Affected annotations:

• @param array<string, string>|null $arguments in the constructor
• @return array{name: string, arguments?: array<string, string>} in getParams()

[chr-hertel]

Thanks @vjik - good catch!