fix(acp): surface agent authentication failures

[purybr365]

Summary

• classify adapter-reported authentication failures centrally in AcpError
• surface terminal credential failures immediately through the existing threaded failure-notice path
• avoid silently retrying the same credential failure while preserving retry behavior for unrelated adapter errors

Problem

An ACP adapter can initialize successfully and later return a generic JSON-RPC error such as -32000: Authentication required while processing a prompt. The harness previously treated that as transient, requeued it with exponential backoff, and produced no channel-visible explanation while the process remained alive.

ACP does not currently define one universal authentication error code, so the shared ACP error layer recognizes Buzz's typed -32001 code plus common adapter-agnostic authentication messages. Queue orchestration only consumes that classification and reuses the existing signed, threaded failure-notice transport.

Validation

• CARGO_INCREMENTAL=0 just ci
• cargo test -p buzz-acp (584 unit tests, 9 integration tests, doc tests)
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

Checklist

• Changes are focused and use the existing ACP error and failure-notice architecture
• Authentication and negative retry-policy cases are covered by tests
• Full repository CI gate passes
• No public API or endpoint documentation changes are required

[purybr365]

Closing as superseded by #2751, which was merged in commit 0a9c26e and implements the same authentication-error handling: classify auth failures, avoid futile retries, surface a user-facing re-authentication notice, and cover the behavior with regression tests. Thanks!