refactor: Replace go-difflib with go-udiff and extract UnifiedDiff to pkg/udiff

[Kybxd]

Summary

Replace the deprecated pmezard/go-difflib dependency with the lighter aymanbagabas/go-udiff library for generating unified diffs, and extract the UnifiedDiff function from generated code into a reusable pkg/udiff package.

Changes

• Extract UnifiedDiff to pkg/udiff package: Moved the UnifiedDiff function out of the generated util.pc.go template into a standalone pkg/udiff package, making it reusable and independently testable.
• Replace diff library: Switched from pmezard/go-difflib to aymanbagabas/go-udiff, which provides a simpler API and eliminates the need to manually split lines.
• Add comprehensive unit tests: Added tests covering no-difference, with-difference, nil/empty messages, and complex nested structpb messages with precise unified diff line assertions.
• Remove golang.org/x/exp dependency: Replaced golang.org/x/exp/slices with the standard library slices package (Go 1.21+).
• Fix comment typo: Corrected mutableCheck comment from "mutable" to "mutated".