Warn about unknown diagnostic rule names after config parsing

[mattiamanzati]

Unknown names in diagnosticSeverity are currently accepted silently. Report them as tsconfig warnings after compiler options have been fully parsed and merged, covering the top-level severity map and overrides.

For example, this plugin configuration now reports warning TS377134: Unknown Effect diagnostic rule 'floatingEfect' in 'diagnosticSeverity'. effect(unknownRuleName):

{
  "name": "@effect/language-service",
  "diagnosticSeverity": {
    "floatingEfect": "error"
  }
}

Configure the diagnostic through diagnosticSeverity.unknownRuleName; it defaults to warning, supports error and off, and honors inherited settings and diagnostics: false. Warnings follow the existing Effect CLI exit-code policy.

Validation reads the resolved Effect options through one post-merge callback, with matching patches for both compiler providers. Source syntax is used only for diagnostic placement: local keys are underlined, while inherited keys without local syntax and JSON-only configurations produce locationless diagnostics, following the content-mapper pattern. Expose the existing generic ForEachTsConfigPropArray through a shared forwarding shim.

Tests cover local and override keys, known diagnostic names, severity controls, chained and multiple extends, existing compiler options, JSON-only parsing, and project references sharing a cached base configuration. Includes a minor changeset.

Validation passed: pnpm setup-repo, pnpm lint (zero issues; deadcode clean), pnpm check, and the full test suite on both TypeScript 7.0.2 and next (Go and shim-generator tests; all 128 CLI tests on each provider). Also checked -p and repeated --build runs with warning, error, and disabled configurations.

The JSON config parser has matching compatibility shims for both providers: 7.0.2 accepts the extra file extensions argument, while the newer compiler omits it. The JSON-only regression test uses the compiler's ordered-map representation, which both versions support.

Supersedes the implementation in #748 with the single post-merge validation approach. Spelling suggestions are not included; inherited declarations use the locationless fallback described above.

Closes #747
Closes #748