[Claude Code - Stage 1] Transparent /v1/messages proxy + protocol contract

[ashwing]

Part of #113 (Claude Code integration).

Transparent proxy for /v1/messages and /v1/messages/count_tokens to the vLLM upstream, plus the Anthropic gateway protocol contract obligations a naive proxy gets wrong.

The proxy itself landed in #99 (raw body/headers/query/status/SSE/error pass-through, dual auth). This issue tracks the remaining contract items that need explicit test coverage:

• system attribution-block strip works — only holds if system is forwarded verbatim, block-first (reordering/merging/stringifying breaks it and pollutes the prompt-cache key).
• anthropic-beta forwarded as an open list (never allowlist beta values — the set changes per release).
• anthropic-version forwarded unchanged.
• Dual auth: accept either Authorization or x-api-key (done in feat: add Anthropic Messages gateway proxy #99).
• Error bodies forwarded unmodified — Claude Code's auto-retry matches on upstream wording (done in feat: add Anthropic Messages gateway proxy #99, worth a regression test).

Acceptance: #99 merged + the two attribution/beta contract tests land. Contract details in #98 → "Gateway protocol contract".