fix(toolkit-lib): guard against a stack with no metadata in countAssemblyResults

[sharonyajain]

countAssemblyResults counts annotation error codes after synth by calling Object.values(stack.metadata) for every stack in assembly.stacksRecursively, with no null guard. When a synthesized stack has no metadata (stack.metadata is undefined), Object.values(undefined) throws:

TypeError: Cannot convert undefined or null to object
    at Object.values (<anonymous>)
    at countAssemblyResults (packages/@aws-cdk/toolkit-lib/lib/toolkit/private/count-assembly-results.ts:11)
    at synthAndMeasure (.../toolkit.ts)
    at Toolkit.synth (.../toolkit.ts)

This crashes Toolkit.synth() for otherwise-valid apps. It was reported downstream in AWS Amplify Gen 2, where a backend defining auth + a multi-model data schema + storage produces at least one nested stack whose metadata is undefined: aws-amplify/amplify-backend#3316.

This change guards the access with stack.metadata ?? {} (one line) and adds a regression test covering both the undefined-metadata case and the normal annotation-counting path.

Fixes #

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[mrgrain]

Suggested change

	import { countAssemblyResults } from '../../../lib/toolkit/private/count-assembly-results';
	import { countAssemblyResults } from '../../../src/toolkit/private/count-assembly-results';