feat(anthropic): add Citations API support

[sobychacko]

Implement Anthropic's Citations API to enable Claude to reference
specific parts of provided documents when generating responses.

New classes:

• Citation: Represents citation metadata with three location types (CHAR_LOCATION, PAGE_LOCATION, CONTENT_BLOCK_LOCATION)
• CitationDocument: Builder for creating citation-enabled documents supporting plain text, PDF, and custom content blocks

Implementation changes:

• AnthropicChatModel: Extract citations from API responses and aggregate via CitationContext into response metadata
• AnthropicChatOptions: Add citationDocuments configuration field
• AnthropicApi: Add CitationResponse and CitationsConfig records

Design decisions:

• Static factory methods for Citation instantiation
• Response-level metadata only (no message-level storage)
• Opt-in behavior with citationsEnabled defaulting to false
• Defensive logging for unexpected citation object types

Testing:

• Integration tests for plain text, PDF, custom content, and multiple document scenarios

Documentation:

• Reference documentation in anthropic-chat.adoc with usage examples, real-world use cases, and best practices

Supported on Claude 3.7 Sonnet and Claude 4 models.

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