feat: extractAt ignores unknown JSON fields by default

enables partial extraction from JSON with extra fields, which is common
when parsing external API responses (e.g., TAP firehose messages have
live, rev, cid fields we don't need).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

src/internal/json.zig

···
             },
         };
     }
-    return std.json.parseFromValueLeaky(T, allocator, current, .{}) catch |err| {
+    return std.json.parseFromValueLeaky(T, allocator, current, .{ .ignore_unknown_fields = true }) catch |err| {
         log.debug("extractAt: parse failed for {s} at path {any}: {s} (json type: {s})", .{
             @typeName(T),
             path,
···
     const result = extractAtOptional(Thing, arena.allocator(), parsed.value, .{ "data", "missing" });
     try std.testing.expect(result == null);
 }
+
+test "extractAt ignores unknown fields" {
+    // real-world case: TAP messages have extra fields (live, rev, cid) that we don't need
+    const json_str =
+        \\{
+        \\  "record": {
+        \\    "live": true,
+        \\    "did": "did:plc:abc123",
+        \\    "rev": "3mbspmpaidl2a",
+        \\    "collection": "pub.leaflet.document",
+        \\    "rkey": "xyz789",
+        \\    "action": "create",
+        \\    "cid": "bafyreitest"
+        \\  }
+        \\}
+    ;
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+
+    const parsed = try std.json.parseFromSlice(std.json.Value, arena.allocator(), json_str, .{});
+
+    // only extract the fields we care about
+    const Record = struct {
+        collection: []const u8,
+        action: []const u8,
+        did: []const u8,
+        rkey: []const u8,
+    };
+
+    const rec = try extractAt(Record, arena.allocator(), parsed.value, .{"record"});
+    try std.testing.expectEqualStrings("pub.leaflet.document", rec.collection);
+    try std.testing.expectEqualStrings("create", rec.action);
+    try std.testing.expectEqualStrings("did:plc:abc123", rec.did);
+    try std.testing.expectEqualStrings("xyz789", rec.rkey);
+}