refactor: migrate Anthropic provider to @ai-sdk/anthropic (#11287)

* refactor: migrate Anthropic provider to @ai-sdk/anthropic

Replace the raw @anthropic-ai/sdk implementation with @ai-sdk/anthropic
(Vercel AI SDK) for consistency with other providers (Bedrock, DeepSeek,
Mistral, etc.).

Changes:
- Replace Anthropic() client with createAnthropic() from @ai-sdk/anthropic
- Replace manual stream parsing with streamText() + processAiSdkStreamPart()
- Replace client.messages.create() with generateText() for completePrompt()
- Use convertToAiSdkMessages() and convertToolsForAiSdk() for format conversion
- Handle prompt caching via AI SDK providerOptions (cacheControl on messages)
- Handle extended thinking via providerOptions.anthropic.thinking
- Add getThoughtSignature() and getRedactedThinkingBlocks() for thinking
  signature round-tripping (matching Bedrock pattern, improves on original
  which had a TODO for this)
- Add isAiSdkProvider() returning true
- Update tests to mock @ai-sdk/anthropic and ai instead of raw SDK

* fix: address PR review - remove apiKey fallback and use system+systemProviderOptions pattern

Author: daniel-lxs