fix(eslint): honor unused directive reporting in flat config

[charan-rathore]

Description

The ESLint checker accepts --report-unused-disable-directives, but its flat-config option translator drops that flag. If a project's flat config disables unused-directive reporting, supplying the flag in lintCommand still produces no diagnostics in dev mode.

Translate the flag into overrideConfig.linterOptions.reportUnusedDisableDirectives. An explicit true sets error severity; false disables reporting. When the flag is absent, keep the project's setting unchanged.

The regression test starts the built ESLint worker with a real flat config and an unused eslint-disable comment. It disables reporting in the config, enables it through the CLI flag, and expects an error diagnostic. That test fails before this fix and passes after it. A translator control checks the absent, false, and true cases.

Validation: all 43 unit tests, package typecheck, production build, and changed-file Biome checks pass. Repository-wide lint has an existing formatting error in untouched utils.spec.ts.

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commit: a151397

[charan-rathore]

Closing for now since nothing has moved here. Happy to pick it back up if a maintainer wants it.