Support Option values in index filters

[Mr-Dust0]

Summary

• Add `FilterableValue` support for `Option` where `T` is already filterable.
• Add Rust compile coverage for filtering option-valued btree indexes by `None`, `Some`, and option ranges.
• Normalize TypeScript table-cache btree comparisons so `null` and `undefined` both behave as option-none.
• Add C# source-generator coverage for nullable btree index accessors.

Fixes #4824.

Test Plan

• `cargo check -p module-test`
• `pnpm --dir crates/bindings-typescript test table_cache_resolved_indexes.test.ts`
• `dotnet test crates/bindings-csharp/Codegen.Tests/Codegen.Tests.csproj --filter NullableBTreeIndexesCompile --no-restore`