refactor(extensions)!: centralize argument extraction

[wackywendell]

Description

This PR simplifies the interface around ArgsExtractor, and pushes the assertions around using all arguments (check_exhausted()) out of user code and into the registry conversion.

fn from_args(args: &mut ArgsAccess<'_>) -> Result<Self, ExtensionError> {
    let path = args.expect_named::<&str>("path")?.to_owned();
    let batch_size = args.get_named::<i64>("batch_size")?.unwrap_or(1024);
    Ok(Self { path, batch_size })
}

Changes

• Changes Explainable::from_args to receive &mut ArgsAccess<'_>.
• Removes ExtensionArgs::extractor, public ArgsExtractor::check_exhausted, and the panic-based Drop assertion.
• Rejects unhandled named and positional arguments after successful decoding.
• Makes positional() mark the complete positional list as handled.
• Keeps output_columns() read-only and outside exhaustion checking because output columns are relation metadata consumed independently by the structural parser.
• Preserves Explainable::to_args(&ExtensionContext) and context-aware relation formatting from feat(extensions)!: Add context to to_args in Explainable trait #202.
• Rewrites the API documentation around the user-visible decoding lifecycle rather than presenting ArgsExtractor as an optional convenience.

API impact

This is a breaking public API change. External Explainable implementations must:

• change from_args(&ExtensionArgs) to from_args(&mut ArgsExtractor<'_>);
• use the extractor passed by the registry;
• remove explicit check_exhausted() calls.

There is intentionally no new public direct-conversion helper. The registry owns the checked decoding boundary.

Validation

• cargo test --all-features
• just check
• Independent review found no actionable issues

[edouardmulliez]

good refacto!