Add input validation for waitUntil and size limits for headerTemplate/footerTemplate

- Add waitUntil validation with allowed values: load, domcontentloaded, networkidle0, networkidle2
- Add size limit validation for headerTemplate and footerTemplate (100KB max)
- Follow TDD approach: 15 new failing tests, then implementation
- All 462 tests passing (was 447)

src/utils/pdf-options.ts

@@ -2,6 +2,8 @@ const VALID_FORMATS = ["Letter", "Legal", "Tabloid", "Ledger", "A0", "A1", "A2",
 const FORMAT_MAP = new Map(VALID_FORMATS.map(f => [f.toLowerCase(), f]));
 const PAGE_RANGES_RE = /^\d+(-\d*)?(\s*,\s*\d+(-\d*)?)*$/;
 const MARGIN_KEYS = new Set(["top", "right", "bottom", "left"]);
+const VALID_WAIT_UNTIL = ["load", "domcontentloaded", "networkidle0", "networkidle2"];
+const MAX_TEMPLATE_SIZE = 102400; // 100KB in characters
 
 type PdfInput = Record<string, any>;
 type ValidResult = { valid: true; sanitized: Record<string, any> };
@@ -79,9 +81,37 @@ export function validatePdfOptions(opts: PdfInput): ValidResult | InvalidResult
     sanitized.pageRanges = opts.pageRanges;
   }
 
-  // Pass through non-validated fields
-  for (const key of ["headerTemplate", "footerTemplate"]) {
-    if (opts[key] !== undefined) sanitized[key] = opts[key];
+  // waitUntil
+  if (opts.waitUntil !== undefined) {
+    if (typeof opts.waitUntil !== "string") {
+      return { valid: false, error: "waitUntil must be a string" };
+    }
+    if (!VALID_WAIT_UNTIL.includes(opts.waitUntil)) {
+      return { valid: false, error: `waitUntil must be one of: ${VALID_WAIT_UNTIL.join(", ")}` };
+    }
+    sanitized.waitUntil = opts.waitUntil;
+  }
+
+  // headerTemplate
+  if (opts.headerTemplate !== undefined) {
+    if (typeof opts.headerTemplate !== "string") {
+      return { valid: false, error: "headerTemplate must be a string" };
+    }
+    if (opts.headerTemplate.length > MAX_TEMPLATE_SIZE) {
+      return { valid: false, error: "headerTemplate must not exceed 100KB" };
+    }
+    sanitized.headerTemplate = opts.headerTemplate;
+  }
+
+  // footerTemplate
+  if (opts.footerTemplate !== undefined) {
+    if (typeof opts.footerTemplate !== "string") {
+      return { valid: false, error: "footerTemplate must be a string" };
+    }
+    if (opts.footerTemplate.length > MAX_TEMPLATE_SIZE) {
+      return { valid: false, error: "footerTemplate must not exceed 100KB" };
+    }
+    sanitized.footerTemplate = opts.footerTemplate;
   }
 
   return { valid: true, sanitized };