help: Restore broken link checks for help center documentation.

We add a step to build help center and then test the broken links as we
used to before removing the test temporarily.

This commit focuses on just adding back the broken link checks for the
help center. We skip the fragment check since that is in-built in
starlight and starlight tests account for that already. For the image
check we can add it back in a followup issue.

zerver/lib/test_helpers.py

@@ -570,15 +570,6 @@ def write_instrumentation_reports(full_suite: bool, include_webhooks: bool) -> N
 
         untested_patterns -= exempt_patterns
 
-        # A lot of help/ URLs are part of the untested patterns, which
-        # is due to the switch to the new help center. We exempt them
-        # programmatically below.
-        untested_patterns = {
-            untested_pattern
-            for untested_pattern in untested_patterns
-            if not untested_pattern.startswith("help/")
-        }
-
         var_dir = "var"  # TODO make sure path is robust here
         fn = os.path.join(var_dir, "url_coverage.txt")
         with open(fn, "wb") as f:
@@ -587,7 +578,7 @@ def write_instrumentation_reports(full_suite: bool, include_webhooks: bool) -> N
         if full_suite:
             print(f"INFO: URL coverage report is in {fn}")
 
-        if full_suite and untested_patterns:  # nocoverage -- test suite error handling
+        if full_suite and len(untested_patterns):  # nocoverage -- test suite error handling
             print("\nERROR: Some URLs are untested!  Here's the list of untested URLs:")
             for untested_pattern in sorted(untested_patterns):
                 print(f"   {untested_pattern}")