feat: add Claude models to the google_cloud provider

[cloetzi]

Stacked on #482. Adds Claude models on Vertex AI to the google_cloud provider, and a shared Riffer::Wire::Anthropic library for building Anthropic requests and parsing responses without the anthropic gem.

Why our own transport

Anthropic::VertexClient only picks up ambient Application Default Credentials and has no way to pass in credentials, so it can't use the fiber-safe token refresh from #482. Claude requests go through the same GoogleCloud::Client as Gemini instead.

Routing

Model ids starting with claude- go to publishers/anthropic/models/{model}:rawPredict and :streamRawPredict, using the Anthropic Messages body plus anthropic_version: "vertex-2023-10-16". Every other model id goes to Gemini. Model ids may contain @ (e.g. claude-haiku-4-5@20251001).

Riffer::Wire::Anthropic

None of these classes depend on the google_cloud provider, so they can be reused if we replace the anthropic gem:

• Request: the hash-building code moved out of the Anthropic provider, which now calls it. Existing Anthropic cassettes pass unchanged.
• Response / Stream: parse raw JSON and SSE events into riffer messages and stream events. Stream raises IncompleteStreamError when there's no message_stop. Replaying all the existing Anthropic cassettes through these classes gives the same results as the SDK path (19 streams, 31 responses).
• Out of scope here: the Anthropic provider still uses the SDK for responses and streaming. Moving it onto Response / Stream is a follow-up.

Other changes

• Client#handle_api_error! handles streamRawPredict errors, which come back wrapped in a JSON array.
• Streamed Claude tool-call deltas carry the real id and name. The SDK path sends "pending" and nil.

Tests

Cassettes are recorded against claude-haiku-5-5: generate, stream, tool calling (generate and stream), and reasoning replay with adaptive thinking.

Commented out: the structured output and web search tests. Our org policy constraints/vertexai.allowedPartnerModelFeatures blocks those features on the recording project. They need an org-policy admin to allow publishers/anthropic/models/claude-haiku-5-5:structured_outputs and :web_search, then the cassettes can be recorded.

Commits

1. Extract Riffer::Wire::Anthropic::Request
2. Add Riffer::Wire::Anthropic response and stream parsers
3. Add Claude models to the google_cloud provider
4. Docs

🤖 Generated with Claude Code

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