Unknown per-language limit key is reported without its languages.<name> path

[ElbertePlinio]

Found during the QA pass for #40, and present on main at 99a0826 (v0.4.0). #40 does not change it.

A .pickcheck.json of

{"languages":{"go":{"limits":{"widgetdepth":1}}}}

fails with

error: unknown config key `limits.widgetdepth` in .../.pickcheck.json

The key actually lives at languages.go.limits.widgetdepth. With several languages configured, the message does not say which block holds the typo. validate_language_keys in crates/core/src/config.rs calls nested_keys(object, "limits", ...), and nested_keys passes only "limits" as the prefix.

Acceptance

• The error names languages.<name>.limits.<key>.
• Top-level limits.<key> errors are unchanged.
• A test covers the per-language case.