examples: add tool use, multi-turn, thinking, and vision examples

[SNO7E-G]

Summary

This PR adds four missing fundamental API usage patterns to the examples/ directory. While the SDK supports these features natively, developers lacked clear, runnable reference implementations for how to orchestrate them in PHP.

Added Examples

1. messages_tool_use.php — Demonstrates a complete agentic loop. It shows how to define tools via ToolUnionShape arrays, detect tool_use blocks in the response, execute functions locally, and feed the tool_result data back to the model. Note: The implementation pattern for appending $response->content to the history mirrors the SDK's internal BetaToolRunner approach.
2. messages_multi_turn.php — Demonstrates conversation history accumulation, showing how to alternate user and assistant turns while preserving a system prompt across requests.
3. messages_thinking.php — Demonstrates extended thinking mode. Crucially, it models both ThinkingConfigEnabled (with an explicit budget) and the recommended ThinkingConfigAdaptive approach, aligning directly with the SDK's deprecation warnings for non-adaptive thinking with Opus 4.
4. messages_vision.php — Demonstrates vision capabilities using both URL-based (URLImageSource) and Base64-encoded (Base64ImageSource) image blocks mixed with text blocks.

Validation

• ✅ All four files pass php -l syntax checks
• ✅ Example usage models exactly align with MessagesService::create() signatures and BaseModel JSON serialization
• ✅ Model constants are consistent with existing examples (using claude-sonnet-4-5-20250929)
• ✅ Code style follows the same formatting conventions (shebang, autoloader placement, and PHPDoc usage) as existing examples like messages.php and messages_stream.php

These examples should significantly improve the onboarding experience for developers building modern agentic or multi-modal applications with the PHP SDK.