feat(model): add provider-native prompt caching

[guslegend0510]

Summary

• Establish shared prompt-cache contracts through GenerateOptions.cacheControl and MessageMetadataKeys.CACHE_CONTROL.
• Add provider-native prompt caching support:
  • OpenAI automatic and explicit caching protocols.
  • DashScope native and OpenAI-compatible cache markers.
  • Anthropic automatic and explicit cache breakpoints.
  • Gemini implicit caching and existing cachedContent resource references.
• Normalize cache-read and cache-write token usage across streaming and non-streaming responses.
• Propagate cache usage through Core, ReActAgent, AG-UI, and Studio.
• Add English and Chinese provider behavior documentation.
• Preserve safe behavior for unknown OpenAI-compatible endpoints and unsupported Ollama cache controls.

Validation

• Core and all five model provider test suites passed.
• AG-UI and Studio test suites passed.
• Spotless and git diff --check passed.
• Documentation build completed successfully.

Gemini cache resource creation, TTL updates, and deletion remain managed by the Google SDK because they have an independent, potentially billable lifecycle.

Closes #2834

[codecov]

Codecov Report

❌ Patch coverage is 70.30016% with 188 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ns/model/openai/formatter/OpenAIBaseFormatter.java	74.21%	16 Missing and 17 partials ⚠️
...thropic/formatter/AnthropicPromptCacheSupport.java	30.95%	21 Missing and 8 partials ⚠️
...anthropic/formatter/AnthropicMessageConverter.java	30.55%	22 Missing and 3 partials ⚠️
...el/dashscope/formatter/DashScopeChatFormatter.java	73.52%	10 Missing and 8 partials ⚠️
...scope/extensions/model/openai/OpenAIChatModel.java	51.61%	7 Missing and 8 partials ⚠️
...extensions/model/dashscope/dto/DashScopeUsage.java	50.00%	7 Missing and 4 partials ⚠️
...hscope/formatter/DashScopeMultiAgentFormatter.java	64.28%	7 Missing and 3 partials ⚠️
...scope/extensions/model/openai/dto/OpenAIUsage.java	62.50%	8 Missing and 1 partial ⚠️
...el/anthropic/formatter/AnthropicBaseFormatter.java	77.14%	3 Missing and 5 partials ⚠️
...scope/extensions/model/gemini/GeminiChatModel.java	33.33%	7 Missing and 1 partial ⚠️
... and 8 more

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