Dependency + toolchain refresh and bug fixes.

- Target Elixir 1.20 / Erlang OTP 28; raise minimum Elixir to ~> 1.18.
- Bump all dependencies to latest (telemetry, opentelemetry*, jason, req_llm,
  jido, ex_doc, dialyxir, credo).
- Fix: render multimodal content parts (:file/:image/:audio) in span
  attributes instead of dropping them.
- Fix: normalize_model_string/1 had an unreachable provider+model clause, so
  provider-tagged models lost their "provider:model" formatting.
- Set package maintainer metadata.