tidy(testutils): Relocate CompareResult

[Cobalt0s]

Description

This PR refines and generalizes the recently introduced CompareResult.

Key changes

• Package relocation
  The CompareResult type has been moved from mockutils to testutils.
  While mockutils is intended for provider-specific mocking utilities, CompareResult is a more general-purpose testing construct.

• Improved AddDiff API
  CompareResult.AddDiff previously required preformatted messages (commonly via fmt.Sprintf).
  It now accepts a format string with variadic arguments, simplifying usage and reducing boilerplate.

• Renamed assertion methods
  The main methods are now named Assert and AssertErr.
  Their return semantics have been updated to align with common testing conventions:

  • true indicates success
  • false indicates failure
    This replaces the previous, less intuitive behavior where true indicated an error.

• Centralized validation logic
  Validation is now fully owned by CompareResult, from instantiation through the Validate stage.
  Calling Validate will invoke t.Fatal(...) if any assertions have failed, making failure handling explicit and easier to follow.

[Cobalt0s]

• feat(codec): Dual marshal preserve RawJSON #2276 : 2 dependent PRs (#2277 , #2876 )
• tidy(testutils): Relocate CompareResult #2944 👈 (View in Graphite)
• main

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[Cobalt0s]

@AmandaYao00 there are some improvements -- tidying of the CompareResult. Let me know if you are ok with the changes and if you have any suggestions to improve anything.

[AmandaYao00]

Looks good!