test: use serial_test to serialize otel process context tests

Replace the hand-rolled `TEST_SERIALIZER: Mutex<()>` with the
`#[serial_test::serial]` attribute applied at the module level, which
is the established pattern already used by other crates in this repo.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: yannham

Cargo.lock

@@ -25,6 +25,7 @@ rmp-serde = "1.3.0"
 
 [dev-dependencies]
 tempfile = { version = "3.3" }
+serial_test = "3.2"
 
 [target.'cfg(unix)'.dependencies]
 memfd = { version = "0.6" }

libdd-library-config/Cargo.toml

@@ -388,6 +388,7 @@ pub mod linux {
     }
 
     #[cfg(test)]
+    #[serial_test::serial]
     mod tests {
         use super::MappingHeader;
         use anyhow::ensure;