Fix Sync Streams json_each object and scalar evaluation

[sravan27]

This fixes a Sync Streams JavaScript evaluator divergence from SQLite around json_each().

SQLite's json_each() emits rows for JSON arrays, JSON objects, and scalar JSON values:

SELECT key, value FROM json_each('{"a":1,"b":true}');
SELECT key, value FROM json_each('42');

The current JavaScript table-valued-function evaluator only accepts arrays, throws for objects, and cannot expose the key column. That means valid Sync Streams rules using object-shaped request/JWT/connection JSON can fail or diverge when evaluated by the JavaScript engine.

This PR:

• Keeps existing array behavior.
• Adds SQLite-compatible rows for JSON objects using object keys.
• Adds SQLite-compatible scalar handling with key = null.
• Exposes both key and value columns from the JavaScript evaluator.
• Adds focused engine coverage for object and scalar json_each() evaluation.
• Adds a patch changeset for @powersync/service-sync-rules.

Verification:

corepack pnpm --filter @powersync/service-sync-rules exec vitest --run test/src/sync_plan/engine/engine.test.ts -t "javascript.*json_each"
corepack pnpm --filter @powersync/service-sync-rules run build:tsc
git diff --check

Note: the local node:sqlite side of this engine test still hits the same local Node binding issue seen on earlier Sync Streams PR verification (column index out of range while binding table-valued JSON inputs), so I scoped the runnable local test command to the JavaScript evaluator path this patch changes.

[changeset-bot]

🦋 Changeset detected

Latest commit: 7a6bae6

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

This PR includes changesets to release 16 packages

Name	Type
@powersync/service-sync-rules	Patch
@powersync/service-core	Patch
@powersync/lib-services-framework	Patch
@powersync/service-module-mongodb-storage	Patch
@powersync/service-module-mongodb	Patch
@powersync/service-module-mssql	Patch
@powersync/service-module-mysql	Patch
@powersync/service-module-postgres-storage	Patch
@powersync/service-module-postgres	Patch
@powersync/service-module-core	Patch
@powersync/service-image	Patch
test-client	Patch
@powersync/service-rsocket-router	Patch
@powersync/lib-service-mongodb	Patch
@powersync/lib-service-postgres	Patch
@powersync/service-schema	Patch

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