test: realign properties generator smoke selectors (#941)

## Summary

- exercise the current click-to-add property-row interaction in SHAFT
smoke tests
- keep custom-property copy coverage on the current scoped Copy control
- preserve TestNG.properties grouping coverage through generated output

Closes #937

## Verification

- focused RED on `origin/master`: 2 tests run, 2 expected stale-selector
failures
- focused GREEN: 2 tests run, 2 passed
- `yarn test:shaft` with
`SHAFT_DOCS_BASE_URL=https://shafthq.github.io`: 27 passed, 0 failures,
0 errors, 0 skipped (confirmed from Surefire XML)
- post-merge focused rerun: 2 passed
- `git diff --check`

## Review

Independent adversarial review of the implementation diff returned zero
blocking and zero non-blocking findings.

Author: MohabMohie

tests/shaft/src/test/java/net/shaftengine/docs/SiteRenderTest.java

@@ -236,13 +236,12 @@ public void propertiesGeneratorCopiesCustomProperties() {
 
         visible(By.xpath("//*[contains(@aria-label,'Generated file destination') and .//*[normalize-space()='src/main/resources/properties/']]"));
         browser.findElement(By.cssSelector("input[placeholder='Property name, section, or description']")).sendKeys("headlessExecution");
-        WebElement row = visible(By.cssSelector("[data-property-key='headlessExecution']"));
-        click(By.cssSelector("[data-property-key='headlessExecution'] input[type='checkbox']"));
+        click(By.cssSelector("[data-property-key='headlessExecution']"));
         selectValue(By.cssSelector("[data-property-key='headlessExecution'] select"), "true");
 
         WebElement output = visible(By.cssSelector("[aria-label='Generated properties files']"));
         wait.until(ExpectedConditions.textToBePresentInElement(output, "headlessExecution=true"));
-        click(By.cssSelector("button[aria-label='Download and copy custom.properties']"));
+        click(By.xpath("//*[@aria-label='Generated properties files']//button[normalize-space()='Copy']"));
         wait.until(driver -> clipboardText().contains("headlessExecution=true"));
     }
 
@@ -251,9 +250,7 @@ public void propertiesGeneratorGroupsTestNgOutputSeparately() {
         navigate("/docs/reference/properties/custom-properties-generator");
 
         browser.findElement(By.cssSelector("input[placeholder='Property name, section, or description']")).sendKeys("setParallel");
-        WebElement row = visible(By.cssSelector("[data-property-key='setParallel']"));
-        SHAFT.Validations.assertThat().object(attribute(row, "data-target-file")).isEqualTo("TestNG.properties");
-        click(By.cssSelector("[data-property-key='setParallel'] input[type='checkbox']"));
+        click(By.cssSelector("[data-property-key='setParallel']"));
         selectValue(By.cssSelector("[data-property-key='setParallel'] select"), "METHODS");
 
         WebElement output = visible(By.cssSelector("[aria-label='Generated properties files']"));