fix: ensure Elixir 1.20 compatibility

[mikehostetler]

Summary

• fix Jido-owned Elixir 1.20 warnings and the test cleanup needed for ExUnit 1.20
• declare optional internal signal extensions explicitly so use Jido.Signal stays warning-free under 1.20
• add GitHub Actions coverage for Elixir 1.20.0-rc.4 on OTP 28, including lint/quality validation

Why

Elixir 1.20 surfaces a few stricter warnings and test-behavior changes that do not show up on the current baseline. This keeps Jido source-compatible on day one and adds CI coverage so we keep verifying that path.

Validation

• mise exec [email protected] -- env MIX_BUILD_ROOT=_build-elixir-1.20-day1-test-clean mix test
• mise exec [email protected] -- env MIX_BUILD_ROOT=_build-elixir-1.20-day1-quality-clean mix q

Notes

• Fresh Elixir 1.20 builds still print warnings from third-party dependencies, but Jido itself is warning-clean and the full quality suite passes.
• This PR supersedes the earlier source-only compatibility branch by including CI coverage too.