fix: report deprecated URI extension fields clearly

[wackywendell]

Description

Closes #96 by replacing the misleading Missing URN anchor 0 failure mode for deprecated URI-based extension fields with explicit formatter diagnostics.

Older hard-coded Substrait JSON can still contain the deprecated extensionUris and extensionUriReference fields. This crate does not normalize those fields into the current URN-based model; instead, textification reports them through the existing formatting-issues channel so callers can fix the JSON input. Relation text still renders best-effort when possible.

This PR also avoids building a fake @0 extension mapping from URI-only declarations, so the old confusing error does not get emitted alongside the clearer deprecated-field diagnostics.

Testing

• RUSTC_WRAPPER= cargo test deprecated_extension_uris -- --nocapture
• RUSTC_WRAPPER= cargo test test_from_extensions_skips_deprecated_uri_only_mappings -- --nocapture
• RUSTC_WRAPPER= just check
• RUSTC_WRAPPER= cargo test --doc
• RUSTC_WRAPPER= just test

Related Issues

Closes #96

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve URI-only mappings for function-name lookup

Dropping DeprecatedUriOnly declarations in from_extensions removes anchor-to-name entries entirely, so deprecated URI-based plans that actually call those functions now lose best-effort textification (NamedAnchor::lookup falls back to MissingAnchor, producing error tokens instead of function names). Before this change, add_extension still inserted the mapping even when it emitted MissingUrn, which preserved readable function names; this regression specifically affects older plans using extensionUriReference plus function calls.

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