[Track/Effect/FileSystem] WriteFile decodes JSON byte arrays via O(N) enum-match loop; empty-path guard duplicated across 4 handlers

[NikolaRHristov]

Problem

1. WriteFile binary decode

FileSystem.WriteFile accepts file content as either a Value::Array of u64 integers or a base64 Value::String. The array path iterates every element with v.as_u64().map(|n| n as u8) — an enum-variant match per byte. For a 50 KB file this is 50,000 matches. For a 200 KB binary (e.g., .wasm written to workspace storage by an extension) this is 200,000 matches. The filter_map also silently drops elements that fail as_u64(), including negative values that TypeScript serializers can produce for bytes >127 via signed Int8Array, causing silent data corruption.

File: Source/Track/Effect/CreateEffectForRequest/FileSystem.rs

2. Empty-path guard duplication

The empty-path if path_str.is_empty() { return Err(...) } guard is copy-pasted identically across ReadFile, WriteFile, ReadDirectory, and Stat with four slightly different error strings. In Stat the guard fires after run_time.Environment.Require() whereas in ReadFile it fires before — inconsistent ordering.

Fix

1. Remove the Value::Array decode path from WriteFile. Require base64 Value::String as the sole accepted encoding. Return a clear error if an array is passed: "FileSystem.WriteFile: content must be base64-encoded string, not byte array".
2. Extract the empty-path guard to a private inline helper:

   #[inline]
   fn require_non_empty_path(method: &str, path: &str) -> Result<(), String> {
       if path.is_empty() {
           Err(format!("{}: empty path (resource not found)", method))
       } else {
           Ok(())
       }
   }

3. Standardize ordering: validate path before run_time.Environment.Require() in all four handlers.

Additional Scope for Same PR

• Add a comment on the vscode://schemas-associations/ special-case in ReadFile referencing the VS Code schema service, so future maintainers understand the origin of this hard-coded prefix.
• Log a follow-up issue: cache FileSystemReader/FileSystemWriter as typed fields on ApplicationRunTime to eliminate the per-call Environment.Require() hash lookup.

[NikolaRHristov]

Fixed: removed the Value::Array decode path in WriteFile (base64-only), added a clear error for byte-array input, reduced empty-path guard duplication with require_non_empty_path, standardized guard ordering before Environment.Require() across ReadFile/WriteFile/ReadDirectory/Stat, and added a clarifying comment for the vscode://schemas-associations special case. Implementation is in Source/Track/Effect/CreateEffectForRequest/FileSystem.rs.