Fix Sync Streams iif arity validation

[sravan27]

Summary

• validate Sync Streams iif() calls with the same three-argument arity used by the evaluator
• add compiler regressions for two-arg and four-arg iif() calls
• add a patch changeset for @powersync/service-sync-rules

Why

iif is implemented and documented as iif(x, y, z), but the compiler previously accepted any call with at least two arguments. That allowed invalid sync definitions such as iif(1, 2) or iif(1, 2, 3, 4) through validation even though the runtime implementation only consumes three arguments.

Verification

• corepack pnpm --filter @powersync/service-sync-rules exec vitest run test/src/compiler/sqlite.test.ts
• corepack pnpm --filter @powersync/service-sync-rules build:tsc

[changeset-bot]

🦋 Changeset detected

Latest commit: d95d0dd

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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