docs: use standalone component pattern in harness guide examples

[yogeshwaran-c]

What kind of change does this PR introduce?

Documentation

What is the current behavior?

The "Using component harnesses" guide uses the legacy declarations + NgModule imports pattern in its TestBed setup examples:

TestBed.configureTestingModule({
  imports: [IssueReportSelectorModule],
  declarations: [IssueReportSelector],
});

This is the NgModule-based testing pattern which is outdated now that Angular defaults to standalone components.

Partially addresses #32709

What is the new behavior?

All three TestBed configuration examples are updated to use the standalone component pattern:

await TestBed.configureTestingModule({
  imports: [IssueReportSelector],
}).compileComponents();

Changes:

• Removed declarations arrays (standalone components are imported, not declared)
• Removed NgModule imports (e.g., IssueReportSelectorModule, MyModule)
• Added await and .compileComponents() to follow current best practices
• Fixed a minor syntax issue (missing closing parenthesis on describe block)

Additional context

The two comparison examples ("without harnesses" and "with harnesses") were both updated to maintain their parallel structure. This is a follow-up to the standalone migration work in PRs #32815, #32825, #32826, and #32827.