test: add long-label injection to TestValidateLabels

[aknuds1]

What this PR does

Following the review comment on PR #14131, this adds long-label injection variants to TestValidateLabels to ensure that long-label handling (truncate/drop strategies) doesn't mask ANY validation error, not just duplicate label detection.

For each eligible test case, the test now runs in three modes:

• no_injection: original behavior
• inject_long_label_truncate: adds a long-value label, uses truncate user
• inject_long_label_drop: adds a long-value label, uses drop user

Test cases are skipped for injection when they would conflict with the injection behavior (already testing long-label handling, non-legacy scheme, would exceed label limits, or have exact label validation).

This provides regression protection: if a validation check is accidentally placed inside the long-label handling branch, the injection variants will catch it by showing the validation error is masked when a long label is present.

Which issue(s) this PR fixes or relates to

Related to #14131

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is ON. A Cloud Agent has been kicked off to fix the reported issue.}

[cursor]

String mutation corrupts expected label values

Low Severity

The hashLabelValueInto function mutates the backing array of injectedLongLabelValue in place. On line 405, after the function call, injectedLongLabelValue[:4] returns "(has" (the first 4 bytes of the hash output) instead of "LLLL". On line 406, the second call computes a hash of the already-mutated string instead of the original. Both injectedLabelTruncatedValue and injectedLabelDroppedValue contain incorrect expected values. This bug is currently masked because these values are only used in dead code.

 

[cursor]

Bugbot Autofix prepared a fix for the bug found in the latest run.

• ✅ Fixed: String mutation corrupts expected label values
  • Used separate string copies for each hashLabelValueInto call to prevent in-place mutation from corrupting the expected values and test input.

Preview (63714cf)

diff --git a/pkg/distributor/validate_test.go b/pkg/distributor/validate_test.go
--- a/pkg/distributor/validate_test.go
+++ b/pkg/distributor/validate_test.go
@@ -402,8 +402,11 @@ func TestValidateLabels(t *testing.T) {
 	// injectedLongLabelValue is 100 chars, exceeding the 75 char limit for truncate/drop users.
 	injectedLongLabelValue := strings.Repeat("L", 100)
 	// Pre-compute the expected hash for the injected long label.
-	injectedLabelTruncatedValue := injectedLongLabelValue[:4] + hashLabelValueInto(injectedLongLabelValue, injectedLongLabelValue)
-	injectedLabelDroppedValue := hashLabelValueInto(injectedLongLabelValue, injectedLongLabelValue)
+	// Use separate copies because hashLabelValueInto mutates the backing array in place.
+	truncateSrc := strings.Repeat("L", 100)
+	injectedLabelTruncatedValue := "LLLL" + hashLabelValueInto(truncateSrc, truncateSrc)
+	dropSrc := strings.Repeat("L", 100)
+	injectedLabelDroppedValue := hashLabelValueInto(dropSrc, dropSrc)
 
 	for _, c := range testCases {
 		caseSchemes := validationSchemes