fix: error on unknown rule names in config

[swwind]

Summary

Unknown, misspelled, or unregistered rule names in rules (e.g. rules: { 'non-existent-rule-name': 'error' }) were silently ignored. The config-time validation step from #1243 now also resolves every enabled rule name, so unknown names are reported alongside rule-options failures. Rules set to "off" are exempt, same as ESLint.

• internal/config: ValidateRuleOptions becomes ValidateRules(config, registry, eslintPlugins) — one pass collects unknown-name errors (unknown rule "x", unknown rule "p/x": plugin "p" has no such rule, unknown rule "p/x": plugin "p" is not registered) and options failures, deduplicated and sorted by rule name. Mounted ESLint-plugin rules resolve through the passed plugin entries, not the registry's placeholders — the placeholders may be unregistered yet at validation time, or stale leftovers from an earlier request in a long-lived process. Declaring a prefix under plugins grants no exemption: enabling a plugin's rules without supplying that plugin's rule names is an error.
• CLI / IPC API: every failure is fatal — the CLI prints all failures and exits 1 before linting starts; the API rejects the request (and assembles its plugin entries before validating, so a metadata-free request can no longer resolve rule names through another request's stale placeholders).
• LSP: failures are downgraded to logged warnings and the offending rules are set to "off" in the committed snapshot; every other rule keeps running and the config transaction commits normally. When the plugin host is degraded the catalog carries no plugin entries, so mounted plugin rules surface as harmless warnings instead of rejecting a valid config.

Breaking Changes

--rule with an unknown rule name now also fails with exit code 1 (previously silently ignored), matching ESLint's --rule behavior. Use "off" if you don't want a rule validated.

Related Links

Fixes #1228

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[swwind]

We might implement this alongside with the new schema validation system. Hold short.