refactor(anthropic): rename translation filters to protocol-keyed names

[leseb]

Summary

Renames the two Anthropic format-translation filters to protocol-keyed names that state the bridge direction: anthropic_to_openai → anthropic_messages_to_chat_completions and anthropic_stream_events → anthropic_messages_to_chat_completions_stream. The rename spans registered filter names, structs, module directories, cross-filter metadata keys (anthropic_to_openai.* → anthropic_messages_to_chat_completions.*, read by time_to_first_token and the stream filter), doctests, error strings, hand-written and generated docs, the example config's filter references, and integration tests. responses_to_chat_completions was already conformant and is unchanged; the example config filename and the stream filter's private anthropic_stream.* per-module state keys are intentionally left as-is (they are not filter identifiers or cross-filter contracts).

Related issue

N/A — naming-clarity refactor, no tracked issue.

Validation

• Unit tests — make test (0 failures)
• Integration or functional tests — make test (updated anthropic_messages suite + examples tests); inference fixtures check-inference green
• make lint (exit 0); make build (exit 0)

Checklist

• I reviewed every changed line and can explain the change.
• New capabilities include an example config and functional example test. (No new capability; existing example config + functional example tests updated for the rename.)
• User-facing behavior and generated documentation are updated. (cargo xtask generate-filter-docs; docs/filters/* and hand-written docs regenerated/updated.)
• Performance-sensitive changes include appropriate benchmark or load-test evidence. (N/A — identifier rename only, no behavior change.)
• Commits are signed and include a Signed-off-by trailer.

Breaking changes

Yes. The old filter names anthropic_to_openai and anthropic_stream_events are removed. Configurations referencing them must migrate to anthropic_messages_to_chat_completions and anthropic_messages_to_chat_completions_stream respectively; any config relying on the anthropic_to_openai.* cross-filter metadata keys must switch to anthropic_messages_to_chat_completions.*.