fix flaky regex resolver e2e tests

[SungJin1212]

This PR fixes flaky regex resolver e2e tests. Previously, the tests waited for cortex_regex_resolver_last_update_run_timestamp_seconds > 0, but this metric can be updated before blocks are uploaded to the bucket, causing the test to proceed with an empty knownUsers list. Instead, we now wait for cortex_regex_resolver_discovered_users == N, which guarantees that the resolver has scanned the bucket and discovered all expected tenants.

    querier_tenant_federation_test.go:226: 
        	Error Trace:	/home/runner/work/cortex/cortex/integration/querier_tenant_federation_test.go:226
        	Error:      	Not equal: 
        	            	expected: model.Vector{(*model.Sample)(0xa733693d620)}
        	            	actual  : model.Vector{}
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1,10 +1,2 @@
        	            	-(model.Vector) (len=1) {
        	            	- (*model.Sample)({
        	            	-  Metric: (model.Metric) (len=1) {
        	            	-   (model.LabelName) (len=8) "__name__": (model.LabelValue) (len=8) "series_1"
        	            	-  },
        	            	-  Value: (model.SampleValue) 0.7491711903904605,
        	            	-  Timestamp: (model.Time) 1778498925016,
        	            	-  Histogram: (*model.SampleHistogram)(<nil>)
        	            	- })
        	            	+(model.Vector) {
        	            	 }
        	Test:       	Test_TenantFederationRegexResolver_WhenSingleTenantMatched

https://github.com/cortexproject/cortex/actions/runs/25666915848/job/75342583581?pr=7462

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags