v0.20.3

Fixed

• Restored the Axum transport adapter in builds by wiring the convert::axum
  module into the crate graph and relaxing the tests to validate responses via
  serde_json::Value instead of requiring ProblemJson deserialization.
• Hardened converter telemetry for Redis, Reqwest, SQLx, Tonic and multipart
  integrations by owning metadata strings where necessary and covering
  non-exhaustive enums so the crate compiles cleanly on Rust 1.90.
• Reworked ProblemJson metadata internals to use Cow<'static, str> keys and
  values, preserving zero-copy behaviour for borrowed data while allowing owned
  fallbacks required by the updated converters.