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

[yamcodes]

This was generated by AI during triage.

Agent Brief

Category: enhancement
Summary: Standardize how validation errors are captured and formatted across all packages

Current behavior:
Different plugins and the core library handle validation errors with slightly different structures and message formats.

Desired behavior:
Centralize the error formatting logic so that whether a user is using Vite, Bun, or the CLI, they see consistent, beautiful error reports.

Key interfaces:

• ArkEnvError class/type — the unified error structure.
• formatError() utility — a shared function for stringifying errors.

Acceptance criteria:

• All packages use the same error normalization logic.
• Error messages include the key name, expected type, and received value.

Out of scope:

• Changing the internal validator logic (e.g., Zod vs ArkType).