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:

---

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.32%. Comparing base (f2f4d60) to head (8f1e39e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1952      +/-   ##
==========================================
- Coverage   91.33%   91.32%   -0.02%     
==========================================
  Files          79       79              
  Lines       12164    12216      +52     
  Branches     1721     1724       +3     
==========================================
+ Hits        11110    11156      +46     
- Misses       1019     1025       +6     
  Partials       35       35              

Flag	Coverage Δ
suite.unit	91.32% <ø> (-0.02%)	⬇️

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