Apply go fix to codebase

[nicholaspcr]

Summary

Apply go fix ./... to modernize the codebase.
https://go.dev/blog/gofix

Changes

• Replace manual for loops with slices.Contains, slices.SortFunc, slices.Clone, etc.
• Replace manual map iteration copies with maps.Copy and maps.Clone
• Replace reflect.TypeOf((*T)(nil)).Elem() with reflect.TypeFor[T]()
• Replace reflect.Ptr with reflect.Pointer
• Replace for i := 0; i < n; i++ with for i := range n (integer range loops)
• Remove ineffective omitempty on struct-typed JSON fields
• Remove unnecessary blank imports and unused assignments

Testing

Steps

Run go build ./... and go test ./... to verify no regressions.

Results

The changes are all mechanical transformations applied by go fix.

Regressions

• The omitempty removal on struct-typed JSON fields is behavior-preserving: omitempty was already a no-op for struct types in encoding/json.
• All other changes (slices, maps, reflect.TypeFor, reflect.Pointer, integer range) are semantically equivalent to the replaced code.

Notes for Reviewers

All changes were generated by go fix ./.... No manual edits.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.