ref/#2540-Move Search Logic from Search.execute() to FhirEngine.search()

[itstanany]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #2540

Description Clear and concise code change description. I moved the search logic implementation from Search.execute to FhirEngine.search For better separation of concerns and single responsibility principles

Alternative(s) considered Have you considered any alternatives? And if so, why have you chosen the approach in this PR?

Type Choose one: Code health

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Checklist

• [x] I have read and acknowledged the Code of conduct.
• [x] I have read the Contributing page.
• [x] I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• [ ] I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• [x] I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project.
• [x] I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally.
• [x] I have built and run the demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).