Adding buck_targets_with_distance.jsonl.zst to list, checking zstd magic number

Summary:
Adding check for buck_targets_with_distance.jsonl.zst. Also adding
a magic number check in case the file extension also isn't correct.

https://fb.workplace.com/groups/td.users/permalink/3848149105481080/

When I ran this from the example:

```
$ arc citrace --verbose --job-id 2981382953332327521 --job-id 3625397700046428817 fbcode//ai_codesign/component_warehouse/tools/automation/test/cogwheel:cogwheel_e2e_geordi-test
```

the artifacts downloaded didn't have extension .zst, but WERE actually zstd:

```
$ file /tmp/citrace/2981382953332327521/buck_targets_with_distance

/tmp/citrace/2981382953332327521/buck_targets_with_distance: Zstandard compressed data (v0.8+), Dictionary ID: None
```

Differential Revision: D92464258

fbshipit-source-id: bbc38407e6b5c712020121cd44dbe3cd91a0e8bc

Author: RiskRunner0

td_util/src/file_io.rs

@@ -19,6 +19,7 @@ use std::path::Path;
 use anyhow::Context;
 
 use crate::json::BUFFER_SIZE;
+use crate::zstd::has_zstd_magic;
 use crate::zstd::is_zstd;
 
 pub fn file_writer(file_path: &Path) -> anyhow::Result<Box<dyn Write>> {
@@ -41,7 +42,7 @@ pub fn file_reader(file_path: &Path) -> anyhow::Result<Box<dyn BufRead + Send>>
     let file = File::open(file_path)
         .with_context(|| format!("Unable to open file `{}` for reading", file_path.display()))?;
 
-    if is_zstd(file_path) {
+    if is_zstd(file_path) || has_zstd_magic(file_path) {
         let decoder = zstd::Decoder::new(file)?;
         Ok(Box::new(BufReader::with_capacity(BUFFER_SIZE, decoder)))
     } else {

td_util/src/json.rs

@@ -25,6 +25,7 @@ use rayon::prelude::*;
 use serde::Deserialize;
 use serde::Serialize;
 
+use crate::zstd::has_zstd_magic;
 use crate::zstd::is_zstd;
 
 /// Buffer size for reading files (10MB)
@@ -38,7 +39,7 @@ fn parse_line<T: for<'a> Deserialize<'a>>(x: Result<String, io::Error>) -> anyho
 
 fn open_file(filename: &Path) -> anyhow::Result<Box<dyn Read + Send>> {
     let file = File::open(filename)?;
-    if is_zstd(filename) {
+    if is_zstd(filename) || has_zstd_magic(filename) {
         Ok(Box::new(zstd::Decoder::new(file)?))
     } else {
         Ok(Box::new(file))

td_util/src/zstd.rs

@@ -10,12 +10,33 @@
 
 //! Utilities for working with zstd compressed files.
 
+use std::fs::File;
+use std::io::Read;
 use std::path::Path;
 
+/// Zstd frame magic number per RFC 8878 Section 3.1.1
+/// https://datatracker.ietf.org/doc/html/rfc8878#section-3.1.1
+const ZSTD_MAGIC: [u8; 4] = [0x28, 0xB5, 0x2F, 0xFD];
+
 /// Check if a file has a zstd extension (.zst)
 pub fn is_zstd(filename: &Path) -> bool {
     match filename.extension() {
         Some(x) => x == "zst",
         None => false,
     }
 }
+
+/// Check if a file contains zstd-compressed data by reading its magic bytes.
+/// Returns true if the file starts with the zstd magic bytes.
+pub fn has_zstd_magic(file_path: &Path) -> bool {
+    let Ok(mut file) = File::open(file_path) else {
+        return false;
+    };
+
+    let mut magic = [0u8; 4];
+    if file.read_exact(&mut magic).is_ok() {
+        magic == ZSTD_MAGIC
+    } else {
+        false
+    }
+}