[benbrandt]

Summary

Addresses SDK-2: malformed parameters could enter ordinary warning logs through Error.data.json and payload-bearing Serde parse reasons.

• Use a shared redacted formatter for protocol-error warnings, emitting only the numeric error code and a fixed, code-derived reason while retaining method/request/handler context where available.
• Apply redaction to typed notification parsing, static/default/dynamic handler failures (including retried messages), outgoing error responses and preparation failures, invalid transport input, and connection-close callback errors.
• Add WARN-level log-capture regressions using a synthetic private marker for missing fields, invalid types, unknown enum variants, and application-controlled error messages/data. Check notification no-reply behavior and continued connection usability.
• Document diagnostic privacy in the book, link it from the TOC, and add a changelog entry.

Privacy boundary

This implements the redacted-WARN formatter option. Returned and wire errors remain unchanged and potentially sensitive, including full failing input JSON. Explicitly enabled DEBUG/TRACE diagnostics remain unchanged. Field paths are not extracted from free-form error text because the current errors do not expose a separately trusted structured path.

Validation

• just test (workspace all-features tests/doctests and core no-default-features tests)
• cargo clippy -p agent-client-protocol --all-features --tests --locked -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• mdbook build after mdbook-mermaid install . (non-blocking preprocessor version mismatch warning)