docs(codecs): thread the connection config in the SchemaCodec example

[MilagrosMarin]

The SchemaCodec class docstring's Example:: block omits config= on both _build_path and _get_backend, so a codec written from it resolves its store against the module-level dj.config rather than the calling connection's. Where a caller holds connections with different store configurations, that config belongs to none of them — the codec either reads the wrong store or, if the global spec carries no usable credentials, fails validation.

The built-in object and npy codecs already read key["_config"] and pass it to both helpers. The example now shows the same, with a note above it explaining why. It also omitted store_name on _build_path, which is where partition and prefix settings come from.

Docs only — the executable AST is unchanged.

Worth considering separately: having _get_backend accept key= and read _config itself would remove the trap rather than document around it. That's an API change, so it isn't here.

[ttngu207]

Approve. Docstring change to the SchemaCodec example that also fixes an omission the plugin bugs didn't happen to hit — the example was missing store_name= on _build_path, which is where partition and prefix settings come from.

Your own note nails the follow-up: "having _get_backend accept key= and read _config itself would remove the trap rather than document around it." Documenting around a trap only helps the next codec author who reads the docstring. Worth filing a separate issue against this repo to change the SchemaCodec._get_backend / _build_path signatures to read _config off key= themselves, so a third-party codec can't silently drop it again. Happy to open that follow-up if you'd like.