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>

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 {