test(agentsessions): the newline case asserts the halves survive, not just the newline

Raised by CodeRabbit.

The comment said the halves "must NOT become the secret" and nothing checked it.
The newline assertion alone passes just as well if the matcher spans the newline
and redacts both halves as one, because the separator would survive inside a
"[REDACTED]" that ate the text around it.

Both halves are now named, and so is the absence of any redaction at all. The
failure this guards against is over-redaction: a credential cannot contain a raw
newline, so treating a newline-split pair as one destroys legitimate transcript
content while protecting nothing. It is also what stops stripControl being
widened to strip newlines, which would make the NUL case in the test above pass
for the wrong reason.

Measured, to pin the three-way distinction:

  newline -> "before sk-ant-api03-\nAAAA... after"   not redacted, halves intact
  joined  -> "before [REDACTED] after"
  NUL     -> "before [REDACTED] after"               stripped, so it rejoins

Mutation-checked: removing the '\t'/'\n' exemption from stripControl fires all
four assertions.

Origin-Session: local-13d543 | Claude Code | 2 prompts
Origin-Snapshot: a939509c08a8

Author: gnanam1990

internal/agentsessions/redaction_order_test.go

@@ -95,8 +95,28 @@ func TestRedactKeepsTheSurroundingText(t *testing.T) {
 	}
 	// A newline separates rather than rejoins, so the halves must NOT become the
 	// secret, and the newline itself is legitimate transcript content.
-	split := redact("before sk-ant-api03-\n" + strings.Repeat("A", 24) + " after")
+	//
+	// THE COMMENT ABOVE WAS THE ONLY THING ASSERTING THE FIRST HALF OF THAT. The
+	// newline check alone passes just as well if the matcher spans the newline
+	// and redacts both halves as one secret — the separator would survive inside
+	// a "[REDACTED]" that ate the text around it. Both halves are named here, and
+	// so is the absence of any redaction at all, because the failure this guards
+	// against is over-redaction: a credential cannot contain a raw newline, so
+	// treating a newline-split pair as one destroys legitimate transcript content
+	// while protecting nothing. It is also what stops stripControl being widened
+	// to strip newlines, which would make the NUL case above pass for the wrong
+	// reason.
+	halves := []string{"sk-ant-api03-", strings.Repeat("A", 24)}
+	split := redact("before " + halves[0] + "\n" + halves[1] + " after")
 	if !strings.Contains(split, "\n") {
 		t.Errorf("a newline was stripped from transcript text: %q", split)
 	}
+	for _, half := range halves {
+		if !strings.Contains(split, half) {
+			t.Errorf("a newline-separated half %q was consumed as part of a secret, leaving %q", half, split)
+		}
+	}
+	if strings.Contains(split, "[REDACTED]") {
+		t.Errorf("two halves separated by a newline were redacted as one secret: %q", split)
+	}
 }