fix(captures): name the violated HAR schema constraint (#2708)

HAR validation reported only the AJV keyword, so a capture missing
`log.version` failed with "HAR schema validation failed: required." and
the caller could not tell which property to supply. Include AJV's
constraint message, which names the missing property, expected type, or
pattern without restating captured values.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/browser/history/HarCapture.test.ts

@@ -162,6 +162,19 @@ it("rejects identical duplicate core fields before schema validation", () => {
   expect(() => decodeHarCapture(text, [])).toThrow("duplicate");
 });
 
+it("names the violated HAR schema constraint without restating capture values", () => {
+  expect(() =>
+    decodeHarCapture(
+      JSON.stringify({
+        log: { creator: { name: "x", version: "1" }, entries: [] },
+      }),
+      [],
+    ),
+  ).toThrow(
+    "HAR schema validation failed: required (must have required property 'version').",
+  );
+});
+
 it("preserves prototype-named producer members through the capture boundary", () => {
   const text = JSON.stringify(historicalHar()).replace(
     '"cache":{}',

src/browser/history/HarSchema.ts

@@ -23,9 +23,13 @@ export const createHarValidator = (): ((value: unknown) => void) => {
     allowMitmproxyAbsentPostData(value);
     if (!validate(value)) {
       const error = validate.errors?.[0];
+      // AJV messages name the violated constraint, such as the missing
+      // property, without restating any captured value.
       throw new CaptureFormatError(
         "format",
-        `HAR schema validation failed: ${error?.keyword ?? "unknown constraint"}.`,
+        error === undefined
+          ? "HAR schema validation failed: unknown constraint."
+          : `HAR schema validation failed: ${error.keyword} (${error.message ?? "no detail"}).`,
         error?.instancePath ?? "",
       );
     }