refactor: simplify Observable creation using of() operator in HistoricalQueryHandler

[Copilot]

Pull Request

🤨 Rationale

Addresses feedback from #517 to replace manual Observable construction with RxJS's idiomatic of() operator in HistoricalQueryHandler.ts.

👩‍💻 Implementation

HistoricalQueryHandler.ts:

• Added of to rxjs imports
• Replaced manual Observable construction pattern with of(aggregatedResults)

// Before
new Observable<TagHistoryResponse>(subscriber => {
    subscriber.next(aggregatedResults);
    subscriber.complete();
})

// After
of(aggregatedResults)

TagDataSource.ts:

• Fixed missing closing parenthesis in switchMap call (syntax error from previous commit)

🧪 Testing

• TypeScript type checking passes
• ESLint validation passes with no new warnings
• CodeQL security scan found 0 alerts

✅ Checklist

• This PR has a title that follows the commit message format.

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