test(rust): fix flaky env-var-dependent client tests (#3426)

The `test_resolve_user_id_*` tests in `remote/client.rs` mutate the
process-global `LANCEDB_USER_ID` and `LANCEDB_USER_ID_ENV_KEY`
environment variables. cargo runs tests in a binary across multiple
threads, so one test's `remove_var` can race another's `set_var` between
when it's set and when `resolve_user_id()` reads it.

This surfaced as an intermittent failure of
`test_resolve_user_id_from_env_key` on Windows CI:

```
assertion `left == right` failed
  left: None
 right: Some("custom-env-user-id")
```

Annotates the five env-mutating tests with `serial_test`'s
`#[serial(user_id_env)]` so they run serially with respect to each
other.

Should be backported to `release/v0.28` (CI for #3421 hit this same
flake).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -5084,6 +5084,7 @@ dependencies = [
  "serde",
  "serde_json",
  "serde_with",
+ "serial_test",
  "snafu 0.8.9",
  "tempfile",
  "test-log",
@@ -8128,6 +8129,15 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "scc"
+version = "2.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "46e6f046b7fef48e2660c57ed794263155d713de679057f2d0c169bfc6e756cc"
+dependencies = [
+ "sdd",
+]
+
 [[package]]
 name = "schannel"
 version = "0.1.29"
@@ -8194,6 +8204,12 @@ dependencies = [
  "untrusted 0.9.0",
 ]
 
+[[package]]
+name = "sdd"
+version = "3.0.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "490dcfcbfef26be6800d11870ff2df8774fa6e86d047e3e8c8a76b25655e41ca"
+
 [[package]]
 name = "sec1"
 version = "0.3.0"
@@ -8384,6 +8400,32 @@ dependencies = [
  "unsafe-libyaml",
 ]
 
+[[package]]
+name = "serial_test"
+version = "3.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "911bd979bf1070a3f3aa7b691a3b3e9968f339ceeec89e08c280a8a22207a32f"
+dependencies = [
+ "futures-executor",
+ "futures-util",
+ "log",
+ "once_cell",
+ "parking_lot",
+ "scc",
+ "serial_test_derive",
+]
+
+[[package]]
+name = "serial_test_derive"
+version = "3.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0a7d91949b85b0d2fb687445e448b40d322b6b3e4af6b44a29b21d9a5f33e6d9"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "sha1"
 version = "0.10.6"

rust/lancedb/Cargo.toml

@@ -104,6 +104,7 @@ datafusion.workspace = true
 http-body = "1"                                        # Matching reqwest
 rstest = "0.23.0"
 test-log = "0.2"
+serial_test = "3"
 
 
 [features]

rust/lancedb/src/remote/client.rs

@@ -888,6 +888,7 @@ pub mod test_utils {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use serial_test::serial;
     use std::time::Duration;
 
     #[test]
@@ -1143,6 +1144,7 @@ mod tests {
     }
 
     #[test]
+    #[serial(user_id_env)]
     fn test_resolve_user_id_none() {
         let config = ClientConfig::default();
         // Clear env vars that might be set from other tests
@@ -1155,6 +1157,7 @@ mod tests {
     }
 
     #[test]
+    #[serial(user_id_env)]
     fn test_resolve_user_id_from_env() {
         // SAFETY: This is only called in tests
         unsafe {
@@ -1169,6 +1172,7 @@ mod tests {
     }
 
     #[test]
+    #[serial(user_id_env)]
     fn test_resolve_user_id_from_env_key() {
         // SAFETY: This is only called in tests
         unsafe {
@@ -1189,6 +1193,7 @@ mod tests {
     }
 
     #[test]
+    #[serial(user_id_env)]
     fn test_resolve_user_id_direct_takes_precedence() {
         // SAFETY: This is only called in tests
         unsafe {
@@ -1206,6 +1211,7 @@ mod tests {
     }
 
     #[test]
+    #[serial(user_id_env)]
     fn test_resolve_user_id_empty_env_ignored() {
         // SAFETY: This is only called in tests
         unsafe {