feat(anthropic): Add prompt caching support

[CeciliaZ030]

Implements Anthropic's prompt caching feature which can reduce costs by up to 90% and latency by up to 85% for repeated prompt prefixes.

Changes:

• Add CacheControl enum with Ephemeral variant
• Add cache_control field to SystemContent, Content, and ToolDefinition
• Add builder methods: .with_cache_control() and .cached()
• Support caching for system prompts, messages, and tool definitions
• Track cache usage in Usage struct (cache_read_input_tokens, cache_creation_input_tokens)

Usage:

// Cache system prompt
let system = SystemContent::text("You are helpful.").cached();

// Cache tool definitions
let tool = ToolDefinition::new("search", "Search the web", schema).cached();

// Cache message content
let content = Content::text("Long context...").cached();

Ref: https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching