fix: normalize empty tool schema properties

[Hanshuk]

Summary

Fixes #405.

PHP serializes empty arrays as [], which causes an empty JSON Schema properties object to be serialized as an array instead of the required JSON object {}.

This can cause strict MCP clients to reject otherwise valid tool definitions.

Changes

• Recursively normalize empty properties arrays to \stdClass.
• Apply schema normalization during Tool::jsonSerialize() so directly constructed Tool instances are handled correctly despite readonly schema properties.
• Apply the same normalization to both inputSchema and outputSchema.
• Preserve legitimate JSON Schema arrays such as required and enum.
• Retain the existing Tool::fromArray() normalization behavior.

Test coverage

Added regression coverage for:

• Direct Tool construction with empty properties.
• Tool::fromArray() with empty properties.
• JSON decode → Tool construction → serialization.
• Nested object schemas.
• Existing non-empty properties.
• Legitimate required and enum arrays.
• outputSchema with empty properties.

Validation

• git diff --check passes.
• PHPUnit could not be executed locally because PHP/Composer are unavailable in the development environment.

The implementation is intentionally limited to the schema normalization and regression tests required for #405.

[chr-hertel]

Hi @Hanshuk, thanks for this, but basically a duplicate of #407, right?
If you have some feedback for that PR to improve it, please share it there.