fix(linter): correct reportUnusedDisableDirectives off behavior, add integration tests, update docstring

[Copilot]

Addresses three issues flagged in the Copilot review of oxc-project#19799.

off/allow not actually disabling unused-directive reporting

WithSeverity(Some(Allow)) was passing through as Some(Allow), which downstream report_unused_directives treated as "report as warning". The same bug existed for the config-store fallback. Now both paths filter Allow → None.

// Before: Allow slipped through and still triggered reporting
ReportUnusedDirectives::WithSeverity(Some(severity)) => Some(severity),
_ => config_store.report_unused_disable_directives(),

// After: only Warn/Deny propagate; Allow (off) disables reporting
ReportUnusedDirectives::WithSeverity(Some(severity)) if severity.is_warn_deny() => Some(severity),
ReportUnusedDirectives::WithSeverity(Some(_)) => None,
_ => match config_store.report_unused_disable_directives() {
    Some(severity) if severity.is_warn_deny() => Some(severity),
    _ => None,
},

Integration tests for config-file path

The existing test only covered the CLI flag path. Two new snapshot tests added:

• test_report_unused_directives_from_config — reportUnusedDisableDirectives: "warn" in .oxlintrc.json reports without any CLI flag
• test_report_unused_directives_cli_overrides_config — CLI --report-unused-disable-directives-severity=off suppresses config-level "warn"

Docstring mentions only eslint-disable-line

Updated to // oxlint-disable-line or // eslint-disable-line to reflect that both prefixes are supported. Propagated to all generated files (config.generated.ts, configuration_schema.json, website snapshots).

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