refactor(go): refactor go sdk errors

[chengxilo]

What I Did

1. Aligned Error Codes and Information with the Rust SDK
2. Refactored IggyError: Discarded Sentinel Errors, Introduced Code(err error) Helper.
3. Updated Tests
4. Removed the Logic for Ignoring xxxAlreadyExists Errors, as referenced in PR #604.

Rationale
Why Discard Sentinel Errors?
Iggy has hundreds of distinct error types, and maintaining a corresponding sentinel variable for each is a nightmare. Since we now standardize on using error codes, extracting them via the Code(err error) helper makes error handling much more scalable and maintainable.
For those who still prefer sentinel errors, I’ve also implemented an Is() function for error comparison, so users can create their own sentinel errors and use them with errors.Is() function.

Why Remove the Logic for Ignoring xxxAlreadyExists Errors?
I believe there’s no need to check for existence before attempting to create something. Instead, I can simply try to create it—if it already exists, the operation will return an error indicating so. I can then handle this error as needed, such as by returning, continuing, or applying other logic.
At least in Go, I don’t think such errors should be ignored by default, it's caller's job to handle it.

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