Fix TypeScript errors under exactOptionalPropertyTypes

Use a local PdfJsXmpMetadata interface instead of the non-exported pdfjs
Metadata type, omit undefined optional fields in reconcileFingerprint, and
type discovery hook mocks with explicit DiscoveryCandidate return types.

src/pdf/metadata.ts

@@ -6,7 +6,10 @@
  */
 
 import { getDocument } from "pdfjs-dist";
-import type { Metadata } from "pdfjs-dist";
+
+interface PdfJsXmpMetadata {
+  getAll(): Record<string, unknown>;
+}
 
 export interface PdfIntrinsicMetadata {
   readonly title?: string;
@@ -72,7 +75,7 @@ export function mergeDocumentFields(
 
 function normalizePdfMetadata(
   info: Record<string, unknown>,
-  metadata: Metadata | null,
+  metadata: PdfJsXmpMetadata | null,
 ): PdfIntrinsicMetadata {
   const result: {
     title?: string;
@@ -172,7 +175,9 @@ function pdfDateField(value: unknown): string | undefined {
   return Number.isNaN(parsed.getTime()) ? trimmed : parsed.toISOString();
 }
 
-function xmpFields(metadata: Metadata | null): Record<string, string> | undefined {
+function xmpFields(
+  metadata: PdfJsXmpMetadata | null,
+): Record<string, string> | undefined {
   if (metadata === null) return undefined;
   const all = metadata.getAll() as Record<string, unknown>;
   const xmp: Record<string, string> = {};

src/recovery/reconcile.ts

@@ -20,7 +20,7 @@ export function reconcileFingerprint(
 
   return {
     unchanged,
-    previousFingerprint,
+    ...(previousFingerprint !== undefined ? { previousFingerprint } : {}),
     currentFingerprint,
     requiresReanchor: !unchanged,
   };

src/recovery/recovery.test.ts

@@ -10,6 +10,7 @@ import {
   reIngestAndCompare,
   recoveryStateAfterLocalSearch,
   searchCanonicalQuote,
+  type DiscoveryCandidate,
 } from "./index";
 
 describe("reconcileFingerprint", () => {
@@ -77,15 +78,19 @@ describe("createSourceDiscoveryRegistry", () => {
     const registry = createSourceDiscoveryRegistry();
     registry.register({
       name: "low",
-      discover: vi.fn(async () => [
-        { uri: "file:///a", confidence: 0.2, source: "local" },
-      ]),
+      discover: vi.fn(
+        async (): Promise<readonly DiscoveryCandidate[]> => [
+          { uri: "file:///a", confidence: 0.2, source: "local" },
+        ],
+      ),
     });
     registry.register({
       name: "high",
-      discover: vi.fn(async () => [
-        { uri: "https://example.com/doc", confidence: 0.9, source: "external" },
-      ]),
+      discover: vi.fn(
+        async (): Promise<readonly DiscoveryCandidate[]> => [
+          { uri: "https://example.com/doc", confidence: 0.9, source: "external" },
+        ],
+      ),
     });
 
     const candidates = await registry.discover({ title: "Example" });