chore: prepare endpoint-libs 2.1.4

[pathscale]

Publishes the merged MCP-only websocket and disconnect-hook work as endpoint-libs 2.1.4 for AgencyProxy.

Dependency cleanup:

• removes the unused public decimal_sf helper and rust_decimal dependency after confirming no callers in endpoint-libs, endpoint-gen, honey_id-types, or the six backend repositories
• removes the unused cert-provider development stack and its obsolete quick-xml chain
• removes all audit ignores
• updates locked ruint to patched 1.20.0

Verification: cargo test --all-features passes 105 tests locally. Cargo.lock contains no rkyv or quick-xml package.