feat(extensions): support impl-level description in extensions

[nielspardon]

Summary

Substrait v0.86.0 added an optional description field to individual function implementations (substrait#1013). An impl-level description documents overload-specific behavior and can appear on each scalar/aggregate/window function implementation.

The variant already inherits description() from Function, so the impl-level value was being parsed but then discarded: resolve() unconditionally overwrote it with the parent function's description. This PR makes the impl-level description take precedence over the parent function's when present, mirroring the impl-level precedence used for deprecation info.

Closes #810

Changes

• Add Function.resolveDescription(...) helper that prefers a non-empty impl-level description over the parent function's, and wire it into the scalar/aggregate/window resolve() methods.
• Add DescriptionExtensionTest and a description_extensions.yaml fixture covering function-level-only (inherited), impl-level overriding function-level, sibling overloads (one overriding, one inheriting), impl-level with no function-level, and aggregate-as-window carry-over.

Testing

• ./gradlew :core:test passes (including the new test).
• ./gradlew :core:spotlessCheck passes.

🤖 Generated with Claude Code

[nielspardon]

Claude thought it makes sense to overwrite the function level description with the impl specific description. I guess this is reasonable.

[vbarua]

Left some minor comments but other looks good to me.

This has the same lossy behaviour as #846 (overwriting the description in some cases), but is consistent with what we do today and we can improve it as a followup.