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`

[bfops]

Hi @Mr-Dust0 , please see the docs here about why this is more work than it may first appear: https://github.com/clockworklabs/SpacetimeDB/pull/4024/changes#diff-3ce7e5c02154d19ebe0624a457ea8b0c30a6b1658a24856ed1ea2ec3ee28916a.

[Mr-Dust0]

Hi @bfops, I’ve updated the PR to cover the additional work you shared in the docs.

The changes I've made add TypeScript module-side support for indexed/unique optional columns, keep non-filterable option types rejected at compile time, and normalize null/undefined option-none comparisons in the TypeScript table cache. I also kept the Rust and C# coverage in place.