fix(cactus): mirror tightened filename length cap in browser variant

Apply the same `MAX_FILENAME_LEN = 251` cap as Cactus_Runtime.ts so a
filename that validates in the browser also validates on Node — the
Node atomic-write path appends `.tmp` (4 chars) and would otherwise
exceed the 255-byte per-component limit on common filesystems.

Author: sroussey

providers/cactus/src/ai/common/Cactus_Runtime.browser.ts

@@ -45,6 +45,11 @@ export interface CactusModelCacheInfo {
 
 const MODEL_ID_RE = /^[A-Za-z0-9_-]{1,64}$/;
 const FILENAME_RE = /^[A-Za-z0-9_.-]+$/;
+// Match the Node variant's limit so an asset that validates here also
+// validates there. The Node atomic-write path writes to `${filename}.tmp`
+// before renaming, so the source filename must leave room for that suffix
+// (most filesystems cap a path component at 255 bytes).
+const MAX_FILENAME_LEN = 251;
 
 function assertSafeModelId(model_id: string): void {
   if (typeof model_id !== "string" || !MODEL_ID_RE.test(model_id)) {
@@ -59,14 +64,14 @@ function assertSafeFilename(filename: string): void {
   if (
     typeof filename !== "string" ||
     filename.length === 0 ||
-    filename.length > 255 ||
+    filename.length > MAX_FILENAME_LEN ||
     filename === "." ||
     filename === ".." ||
     !FILENAME_RE.test(filename)
   ) {
     throw new Error(
       `Invalid Cactus asset filename ${JSON.stringify(filename)}: ` +
-        `must match ${FILENAME_RE} (no path separators, no '..').`
+        `must match ${FILENAME_RE} (no path separators, no '..'), 1-${MAX_FILENAME_LEN} chars.`
     );
   }
 }