Unify error normalization and formatting

[yamcodes]

Refactor the error system to use a normalized EnvIssue format and a consistent formatIssues utility. This will allow better handling of errors from multiple validation engines (ArkType, Standard Schema) and provide a stable base for CLI and IDE integrations.

• Consolidate EnvIssue type.
• Implement stable formatIssues logic.
• Ensure consistent error message reshaping across validators.

Was originally part of #705 but taken out to reduce the PR (#704) size.

[yamcodes]

Might not be neccessary given #722 - keeping an eye on this