test(server): isolate config tests from ambient env via temp-env

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

Cargo.lock

@@ -1614,6 +1614,7 @@ dependencies = [
  "reqwest",
  "serde_json",
  "sqlx",
+ "temp-env",
  "tokio",
  "tracing",
  "tracing-subscriber",
@@ -1974,6 +1975,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "temp-env"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "96374855068f47402c3121c6eed88d29cb1de8f3ab27090e273e420bdabcf050"
+dependencies = [
+ "parking_lot",
+]
+
 [[package]]
 name = "thiserror"
 version = "2.0.18"