[proton0210]

Closes #4747.

Literal external resource identifiers leave remote gathered properties unavailable. The schema resolver previously compared known local values against the unresolved $data object, producing false SQS and RDS findings. Treat missing remote values and deployment-time intrinsic values as unknown comparisons. Preserve that uncertainty through conditional, negated and oneOf schemas, while retaining literal reference comparisons explicitly requested by cfnContext.

Add the reported SQS/RDS reproduction, known cluster mismatch controls, tests for missing and false-like values, and composite-schema regressions. Existing API Gateway and VPC reference-comparison tests remain passing.

Validation: 547 focused tests passed. Three audit passes covered upstream reproduction, full unit/non-regression checks, and final source review with all four independent fixes applied together (3,027 unit/integration tests passed plus the separate schema-updater test; four data tests retain the default exclusion).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.70%. Comparing base (fd17da4) to head (5804f6b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4754      +/-   ##
==========================================
- Coverage   94.71%   94.70%   -0.01%     
==========================================
  Files         439      439              
  Lines       15886    15895       +9     
  Branches     3076     3078       +2     
==========================================
+ Hits        15046    15053       +7     
- Misses        458      459       +1     
- Partials      382      383       +1     

Flag	Coverage Δ
unittests	94.70% <100.00%> (-0.01%)	⬇️

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