feat: rich proto types in ExtensionArgs for Explainable trait (#98)

[wackywendell]

Description

Redesigns the extension argument system so Explainable implementations work directly with parsed Substrait proto types instead of raw strings.

Key changes:

• ExtensionColumn::Named carries proto::Type instead of String — the parser already parsed these, now it keeps the result
• ExtensionValue::Type(proto::Type) — new variant for type arguments in extension relations
• RawExpression replaced with Expr(Box<proto::Expression>) — expressions are now fully parsed
• ExplainContext passed to from_args/to_args, with schema()/columns() helpers for NamedStruct conversion
• Grammar adds extension_type_arg for types in extension arguments (simple + compound types)
• fixtures::parse_type() helper for constructing types from strings in tests

Breaking changes to the Explainable trait:

• from_args(args: &ExtensionArgs) → from_args(args: &ExtensionArgs, ctx: &ExplainContext)
• to_args(&self) → to_args(&self, ctx: &ExplainContext)
• ExtensionColumn::Named { type_spec: String } → { ty: proto::Type }
• RawExpression removed, Expr newtype added
• Registry parse_*/decode_* methods take &SimpleExtensions

Type of Change

• Bug fix
• New feature
• Documentation update
• Example update
• Other (please describe)

Testing

• Added tests for new functionality
• All existing tests pass
• Ran examples to verify behavior

7 new tests cover Type arguments (roundtrip), TryFrom impls, and ExplainContext schema/columns helpers including error cases.

Checklist

• Code follows Rust conventions
• Documentation updated if needed
• No breaking changes (or breaking changes documented)
• Pre-commit hooks pass

Related Issues

Closes #98

[wackywendell]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 940c92f64b

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

[chatgpt-codex-connector]

Accept user-defined types in extension arguments

ExtensionValue::Type now carries arbitrary proto::Type, and textification will emit user-defined types, but the new grammar only permits simple_type | compound_type for extension_type_arg. As a result, a valid top-level user-defined type argument (for example, from an extension’s to_args) cannot be parsed back, so format→parse roundtrips break for extensions that use user-defined type args. This is especially problematic for named args where the ambiguity concern does not apply after =.

Useful? React with 👍 / 👎.