fix(i18n): wrap parse error cause

[resolvicomai]

What?

• Wrap the JSON parser source error with %w instead of rendering it with %v.
• Add regression coverage that preserves both ErrParseFailed and the original json.SyntaxError in the error chain.

Closes #1050

Why?

The i18n parser already wraps ErrParseFailed, but the underlying JSON parse error was converted to text. That made errors.As unable to recover the original cause from callers that need to inspect it.

Tests

• go test ./i18n -count=1 -run TestParseJSONWrapsSyntaxError
• go test ./i18n -count=1
• go test ./...
• make lint
• git diff --check

[FromSi]

lgtm