What

Adds Anthropic document citations end-to-end:

• Request: Content::Document gains optional title, context, and typed citations: CitationsConfig fields. TryFrom<message::Document> forwards additional_params so users going through the generic surface can opt in.
• Response: Content::Text carries typed citations: Vec<Citation> with all three Anthropic location types (char_location, page_location, content_block_location). Conversion to message::AssistantContent preserves them via message::Text.additional_params.
• Streaming: new ContentDelta::CitationsDelta variant. Also added #[serde(other)] Unknown so any future delta type Anthropic adds does not break stream deserialization.
• New public helper anthropic_citations(&message::Text) -> Result<Vec<Citation>, serde_json::Error> for typed extraction from the generic message::AssistantContent surface.

Why

The Messages API exposes citation metadata (RAG, document QA, auditable model output). Rig previously dropped these fields silently both on the request side (when set via message::Document.additional_params) and on the response side (the citation arrays returned on text blocks). This PR closes the gap without forcing other providers to model fields they don't support.

Compatibility note

message::Text gains additional_params: Option<serde_json::Value> mirroring Image/Audio/Video/Document in the same module — Text was the only content type missing this. The change is non-breaking at the JSON wire level (default None, #[serde(flatten, skip_serializing_if = "Option::is_none")]) but downstream code using the struct literal message::Text { text } should migrate to Text::new(text) or add additional_params: None. A constructor is provided. Same generic mechanism would let future providers (logprobs, reasoning notes, ...) attach metadata to text blocks without a new abstraction per provider.

Testing

Full workspace baseline green:

• cargo build --workspace
• cargo test --workspace — 610 tests pass; added 9 in anthropic/completion.rs (document serialization with citations, three location-type round-trips, helper extraction in both empty and populated paths, AssistantContent round-trip preserving citations, end-to-end additional_params forwarding on a request) and 2 in anthropic/streaming.rs (citations_delta event deserialization, unknown-delta fallback).
• cargo clippy --workspace --all-targets --all-features -- -Dwarnings
• cargo +nightly fmt --all --check
• cargo +nightly doc --workspace --all-features --no-deps --document-private-items with RUSTDOCFLAGS="-D warnings"

Notes

• Streaming consumer currently drops CitationsDelta (returns None from handle_event); citations are accumulated per-content-block and returned with the assembled final Content::Text. Real-time citation streaming through RawStreamingChoice is out of scope here and would be a follow-up.
• Citation validation (the API requires all-or-none citations across documents in a request) is left to the server — adding client-side validation is scope creep.

AI assistance disclosure

Implemented with a two-stage AI-assisted process:

1. Claude Opus 4.7 (Anthropic) wrote the implementation, tests, and documentation, following the project's idiomatic Rust style (Result-based error propagation, #[derive(Default)] on the extended message::Text so downstream struct-literal call-sites stay ergonomic).
2. Kimi K2.6 (Moonshot) ran two independent reviews:
   • Pre-coding architectural audit of the implementation plan — caught missing impl From<String>/impl Display updates and the real scope of ~22 call-sites that would break with the new additional_params field before any code was written.
   • Final diff review — caught compile failures in rig-bedrock and rig-vertexai not surfaced by cargo check -p rig-core.

I reviewed every line and take responsibility for correctness and maintainability. Full workspace baseline is green: cargo build, test, clippy --all-targets --all-features -- -Dwarnings, +nightly fmt --check, and +nightly doc -D warnings.

Fixes #1767