fix: Resolve Exception#detailed_message name collision

[jterapin]

Fixes #3383

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Summary

• Fix name collision between generated detailed_message accessor on NeptuneData error classes and Ruby 3.2+ Exception#detailed_message(highlight:, **)
• Generated accessor now uses kwargs-based dispatch: no-arg calls return the API field, calls with keyword arguments (e.g., from full_message) delegate to super
• Adds exception_method flag to error codegen, separate from existing shared flag

Context

NeptuneData is the only AWS service whose error shapes have a detailedMessage member. The generated zero-arg def detailed_message shadowed Ruby 3.2+'s Exception#detailed_message(highlight:, **), causing ArgumentError when full_message is called (triggered by OpenTelemetry, logging frameworks, etc.).

Testing

Tested locally with Ruby 3.0 and Ruby 4.0
Expected code-generated changes: 99ec980

[sichanyoo]

Nice!

[richardwang1124]

LGTM!