anthropic: report cache_read_input_tokens in /v1/messages usage

[aeon-x]

Summary

The /v1/messages (Anthropic Messages API) response omits prefix-cache
accounting. AnthropicUsage already declares cache_read_input_tokens,
and the underlying ChatCompletionResponse carries the count in
usage.prompt_tokens_details.cached_tokens (populated when
--enable-prompt-tokens-details is set), but the converter never copies
it across — so clients always see cache_read_input_tokens: null even on
a warm prefix-cache hit. /v1/chat/completions reports it correctly for
the same request; this brings /v1/messages to parity.

Changes

• messages_full_converter (non-streaming): map
  prompt_tokens_details.cached_tokens → cache_read_input_tokens.
• streaming message_start usage: same mapping, guarded so it stays
  None when token details aren't present (no behavior change when the
  detail is unavailable).

Verification

On a deployed runtime (Qwen3-Coder-30B, --enable-prompt-tokens-details
already set), a warm identical prompt returns:

• /v1/chat/completions → prompt_tokens_details.cached_tokens: 2800
• /v1/messages (before) → {input_tokens, output_tokens} only
• /v1/messages (after) → includes cache_read_input_tokens

Test plan

• Warm a prefix, hit /v1/messages non-streaming, confirm cache_read_input_tokens is populated
• Same for streaming (message_start event usage)
• Cold request still reports cache_read_input_tokens: null (or 0), no regression

[github-actions]

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