leftJoin: whole-object Ref extraction loses nullable type (ResultTypeFromSelect missing IsNullableRef check)

[rena0157]

• I've validated the bug against the latest version of DB packages

Describe the bug

leftJoin results lose their nullable type when selected as whole objects. Individual property access via RefLeaf correctly produces T | undefined, but selecting the entire joined object via Ref<T, true> produces T instead of T | undefined.

This was partially fixed in #1262 for RefLeaf, but the Ref (whole-object) branch in ResultTypeFromSelect was missed.

To Reproduce

const result = useLiveSuspenseQuery(
  (q) =>
    q
      .from({ order: ordersCollection })
      .leftJoin({ dept: departmentsCollection }, ({ order, dept }) =>
        eq(order.deptId, dept.id),
      )
      .select(({ order, dept }) => ({
        order,
        dept, // typed as Department, should be Department | undefined
      })),
  [],
)

// TypeScript says dept is non-nullable, but at runtime it IS undefined
// when no matching department exists
result.data[0].dept.name // no type error, crashes at runtime
result.data[0].dept?.name // linter warns "unnecessary optional chain" — but it's required

Expected behavior

Whole-object refs from leftJoin should be typed as T | undefined, matching the existing RefLeaf behavior introduced in #1262.

Root cause

In src/query/builder/types.ts, ResultTypeFromSelect handles Ref and RefLeaf differently:

// Ref branch (whole object) — MISSING IsNullableRef check
TSelectObject[K] extends Ref<infer _T>
  ? ExtractRef<TSelectObject[K]>

// RefLeaf branch (individual property) — correctly checks IsNullableRef
TSelectObject[K] extends RefLeaf<infer T>
  ? IsNullableRef<TSelectObject[K]> extends true
    ? T | undefined
    : T

ExtractRef calls WithoutRefBrand<T> which strips NullableBrand, so the Nullable = true flag from Ref<T, true> is lost before it can be checked.

Proposed fix

Add the IsNullableRef check to the Ref branch, matching the existing RefLeaf pattern:

- TSelectObject[K] extends Ref<infer _T>
-   ? ExtractRef<TSelectObject[K]>
+ TSelectObject[K] extends Ref<infer _T>
+   ? IsNullableRef<TSelectObject[K]> extends true
+     ? ExtractRef<TSelectObject[K]> | undefined
+     : ExtractRef<TSelectObject[K]>

Desktop

• OS: macOS
• @tanstack/db: 0.6.4
• TypeScript: 6.0

Additional context

• Builds on LEFT JOIN: select() callback receives truthy proxy objects for unmatched rows instead of null/undefined #1251 and PR fix(db): use Ref<T, Nullable> for left join select refs instead of Ref<T> | undefined #1262 which introduced the Ref<T, Nullable> brand pattern
• We're working around this with a patch on @tanstack/db@0.6.4