Fix Ruby 3.1+ Date#to_s deprecation warning in ProcResultCache

[tob1k]

Summary

Fixes a deprecation warning in Ruby 3.1+ where Date#to_s without a format argument is deprecated.

The build_proc_key method was using function_arguments.join('_') which implicitly calls to_s on Date objects. This now explicitly uses iso8601 for Date arguments, which:

• Eliminates the deprecation warning
• Produces consistent, locale-independent cache keys

Changes

• Modified build_proc_key to explicitly format Date arguments using iso8601
• Added test to verify Date argument handling

Test Plan

• Existing tests pass
• New test added for Date argument formatting