server/failtracer: skip self-referential undefined-ref hints

The compile-time fail hints used a fuzzy match against the ref's
top-level segment to suggest typo fixes (e.g. input.frut -> input.fruit).
levenshtein.ClosestStrings returns the exact match itself when the
top-level segment already matches a declared unknown, so any failure
caused by a missing/undefined sub-field (rather than a misspelled
top-level name) produced a hint suggesting the exact same ref back,
e.g. "input.resource.heading undefined, did you mean
input.resource.heading?".

Skip the hint entirely in that case, since the fuzzy matcher has no
visibility into sub-fields and suggesting the ref unchanged is a no-op.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

v1/server/failtracer/failtracer.go

@@ -85,6 +85,13 @@ func (b *failTracer) Hints(unknowns []ast.Ref) []Hint {
 			continue
 		}
 		miss := string(tblPart)
+		if slices.Contains(candidates, miss) {
+			// The top-level segment is already a declared unknown, so the
+			// failure comes from something deeper in the ref that this
+			// fuzzy match can't see. Suggesting the ref itself as a "fix"
+			// would be a no-op, so skip it.
+			continue
+		}
 		rs := ref[1:].String()
 		if _, ok := seenRefs[rs]; ok {
 			continue

v1/server/failtracer/hints_test.go

@@ -84,6 +84,14 @@ func TestHints(t *testing.T) {
 				{Message: "input.fruit.colour undefined, did you mean input.fruits.colour?"},
 			},
 		},
+		{
+			note: "top-level ref already correct, no self-referential hint",
+			evts: []topdown.Event{
+				evtFromExpr(`__local1__ = input.resource.heading`),
+			},
+			unknowns: []string{"input.resource"},
+			exp:      nil,
+		},
 		{
 			note: "same typo, multiple fail events",
 			evts: []topdown.Event{