[Varshith-Kali]

Issue #, if available:
#4822

Description of changes:

E3690 (AWS::RDS::DBCluster) and E3691 (AWS::RDS::DBInstance) rejected Engine values that RDS serves and CloudFormation accepts, producing false positives on valid templates:

• Engine: neptune on both AWS::RDS::DBCluster and AWS::RDS::DBInstance (RDS serves the neptune engine, the resource schemas type Engine as a plain string, and CloudFormation creates the stack)
• Engine: sqlserver-dev-ee and Engine: sqlserver-dev-se on AWS::RDS::DBInstance (both are documented valid values)

Root cause: the engine allowlists are hand-maintained in scripts/update_schemas_from_aws_api.py (_ENGINE_SUPPORTED and the cluster engine lists) rather than derived from what RDS serves, and the three engines were missing. The generated extension schemas (aws_rds_dbinstance/engine_version.json, aws_rds_dbcluster/engine_version.json) inherited the gap.

Changes:

• scripts/update_schemas_from_aws_api.py: added neptune to _ENGINE_SUPPORTED and to the DB cluster engine lists (including the cluster deprecated writer, keeping it in parity with the cluster writer); added sqlserver-dev-ee and sqlserver-dev-se to _ENGINE_SUPPORTED. The DB instance deprecated writer keeps its existing 4-engine subset, matching the established pattern for the other supported engines.
• Regenerated data (hand-applied with the script's exact JSON dump parameters since this environment has no AWS API access): the three engines added to the Engine enum in aws_rds_dbinstance/engine_version.json, neptune added to the Engine enum in aws_rds_dbcluster/engine_version.json. Per-engine EngineVersion entries for the new engines will populate on the next script run with API access; until then EngineVersion is unconstrained for these engines (fail-open, the safe direction).
• Added test/unit/rules/resources/rds/test_db_engine_version_data.py: regression tests that load the real extension schemas and assert the issue's cases pass while unknown engines still fail.

Verification:

• New tests: 5 of 9 fail on the pre-fix tree (the four issue cases plus a mixed-case Neptune case), all 9 pass post-fix.
• Full test/unit/rules/ suite: 2033 passed. ruff check and ruff format --check clean on changed files.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.68%. Comparing base (e8f395b) to head (833f7ed).

Additional details and impacted files

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##             main    #4830   +/-   ##
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  Coverage   94.68%   94.68%           
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  Files         439      439           
  Lines       15973    15973           
  Branches     3097     3097           
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  Hits        15124    15124           
  Misses        467      467           
  Partials      382      382           

Flag	Coverage Δ
unittests	94.67% <ø> (ø)

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