tsdb/prometheus: Add comprehensive unit tests for Service methods

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: The Prometheus datasource (`pkg/tsdb/prometheus/`) currently has minimal test coverage. Only `extendClientOpts` has two test cases, while eight other Service methods lack unit tests. This makes the codebase harder to maintain, refactor safely, and increases the risk of regressions. Without comprehensive tests, future changes could break functionality without being caught early. **What would you like to be added**: Add comprehensive unit tests for the following eight Service methods that currently lack test coverage: 1. `QueryData` 2. `CallResource` 3. `GetBuildInfo` 4. `GetHeuristics 5. `CheckHealth` 6. `ValidateAdmission` 7. `MutateAdmission` 8. `ConvertObjects` Tests should follow Grafana's [backend testing guidelines](https://github.com/grafana/grafana/blob/main/contribute/backend/style-guide.md#testing): - Use standard library `testing` package - Use `testify` for assertions (`require.*` for hard checks, `assert.*` for soft checks) - Use `t.Run` for sub-tests to group test cases - Use `t.Cleanup` for resource cleanup - Follow existing test patterns in the codebase **Who is this feature for?** This is a code quality improvement for maintainers and contributors.