docs: clarify local Jido test helpers

[mikehostetler]

Summary

• clarify that JidoTest.Case / JidoTest.Eventually are repo-local test helpers, not published package APIs
• show an app-owned MyApp.JidoCase pattern for isolated runtime tests
• update related guide/module docs and usage rules to describe the isolation pattern directly

Verification

• mix test
• mix q

Related to #295. Leaving the issue open for any follow-up discussion.

[mikehostetler]

Reviewed the updated diff. This is now docs-only, uses plain ExUnit.Case setup instead of MyApp.JidoCase, and leaves CHANGELOG.md untouched. Local checks passed (mix test, mix q). GitHub CI summary is green; the only red lane is the non-blocking experimental OTP 29 / Elixir 1.20-rc compile job, failing in meck dependency compilation and marked as warning-only.