fix(db): treat objects with Symbol.toStringTag as leaf values in IsPlainObject

[sleitor]

Summary

Fixes #1372

Temporal types (e.g. Temporal.PlainDate, Temporal.ZonedDateTime, Temporal.Instant) have Symbol.toStringTag set to a string. Previously, IsPlainObject would return true for these types because they are objects and not in the JsBuiltIns union. This caused the Ref<T> mapped type to recursively walk their methods and mangle them to {}.

Root Cause

In packages/db/src/query/builder/types.ts, IsPlainObject<T> determines how each field is processed through the Ref<T> mapped type:

• Plain objects → recursively walked to build nested refs
• Non-plain objects (arrays, JsBuiltIns like Date, Map, etc.) → treated as leaf values, type preserved as-is

Temporal.PlainDate extends object but is not in the JsBuiltIns union, so IsPlainObject returns true. This causes Ref<T> to recursively walk its methods, producing:

Type '{ readonly [Symbol.toStringTag]: "Temporal.PlainDate"; equals: {}; ... }' 
  is not assignable to type 'PlainDate'

Fix

Adds a T extends { readonly [Symbol.toStringTag]: string } check to IsPlainObject before returning true. Objects with Symbol.toStringTag are class instances (Temporal types, custom tagged objects), not plain data objects. This covers all Temporal types without requiring an explicit import or a maintained hardcoded list.

type IsPlainObject<T> = T extends unknown
  ? T extends object
    ? T extends ReadonlyArray<any>
      ? false
      : T extends JsBuiltIns
        ? false
        : T extends { readonly [Symbol.toStringTag]: string }
          ? false  // ← new: class instances with toStringTag are leaf values
          : true
    : false
  : false

Testing

Lint and build pass. The type change is entirely in types.ts with no runtime behavior affected.

[changeset-bot]

🦋 Changeset detected

Latest commit: 634beb3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 12 packages

Name	Type
@tanstack/db	Patch
@tanstack/angular-db	Patch
@tanstack/electric-db-collection	Patch
@tanstack/offline-transactions	Patch
@tanstack/powersync-db-collection	Patch
@tanstack/query-db-collection	Patch
@tanstack/react-db	Patch
@tanstack/rxdb-db-collection	Patch
@tanstack/solid-db	Patch
@tanstack/svelte-db	Patch
@tanstack/trailbase-db-collection	Patch
@tanstack/vue-db	Patch

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commit: e29f108