fix: correct apk-parser test expectations and regex pattern

- Use letter-only regex pattern /ft_([a-z]+)_/i to correctly detect environment suffixes
- This ensures version numbers (starting with digits) are treated as no environment suffix
- Update test expectations to match spec behavior:
  - ft_0_42_ff9ff3441.apk now correctly returns 'product' (no match)
  - invalid_filename.apk returns 'product' (no match)
- All 10 tests pass with corrected implementation

lib/apk-parser.ts

@@ -16,8 +16,8 @@ const ENVIRONMENT_MAP: Record<string, ApkEnvironment> = {
 
 export function detectEnvironment(filename: string): ApkEnvironment {
   // Pattern: ft_[environment]_version_commit.apk
-  // Extract the environment segment between first and second underscores
-  const match = filename.match(/^ft_([^_]+)_/);
+  // Extract the environment segment (alphabetic only) between first and second underscores
+  const match = filename.match(/^ft_([a-z]+)_/i);
 
   if (!match) {
     // No environment suffix, default to product