feat: support sampling with tools

[wWzZb]

What

• Add tools and toolChoice to sampling requests and wire them through ClientGateway::sample().
• Add sampling tool-use and tool-result content blocks, array content support, message metadata, and standard stop-reason values with open-string compatibility.
• Expose sampling.context and sampling.tools client capabilities.
• Add round-trip and validation coverage for the new schema types.

Why

The SDK did not yet model the SEP-1577 protocol additions, so servers could not express tool-enabled sampling loops through the typed API.

Impact

Existing single-content sampling calls remain supported. The new constructor parameters are optional and appended to preserve current call sites.

Checks

• make cs
• make phpstan
• make tests — 936 tests, 2781 assertions, 7 existing skips

Closes #155