GetEnvVariable: return std::optional<std::string> (#1017)

- `GetEnvVariable` now returns `std::optional<std::string>` so callers can distinguish an unset variable from one set to an empty value.
- Windows path uses `SetLastError(ERROR_SUCCESS)` + `ERROR_ENVVAR_NOT_FOUND` to detect "not found"; POSIX path returns `nullopt` when `getenv` returns `nullptr`.
- All call sites migrated. Most use `.value_or("")` to preserve current empty-or-unset behavior. The diagnostic helpers in `zen-test/artifactprovider-tests.cpp` now report `<unset>` vs `<empty>` distinctly.
- Added a check in the `ExpandEnvironmentVariables` test confirming `nullopt` for an unset variable; PATH/HOME lookups in that test use `REQUIRE(has_value())` so a missing var fails cleanly instead of throwing `bad_optional_access`.

3 files changed, 8 insertions, 8 deletions

diff --git a/src/zenserver/hub/hydration.cpp b/src/zenserver/hub/hydration.cpp
index 2730ba059..621af8a46 100644
--- a/src/zenserver/hub/hydration.cpp
+++ b/src/zenserver/hub/hydration.cpp
@@ -2303,11 +2303,11 @@ S3Hydration::S3Hydration(const Configuration& Config) : HydrationBase(Config)
 	std::string Region = std::string(Settings["region"sv].AsString());
 	if (Region.empty())
 	{
-		Region = GetEnvVariable("AWS_DEFAULT_REGION");
+		Region = GetEnvVariable("AWS_DEFAULT_REGION").value_or("");
 	}
 	if (Region.empty())
 	{
-		Region = GetEnvVariable("AWS_REGION");
+		Region = GetEnvVariable("AWS_REGION").value_or("");
 	}
 	if (Region.empty())
 	{
@@ -2322,7 +2322,7 @@ S3Hydration::S3Hydration(const Configuration& Config) : HydrationBase(Config)
 		m_PathStyle = Settings["path-style"sv].AsBool();
 	}
 
-	std::string AccessKeyId = GetEnvVariable("AWS_ACCESS_KEY_ID");
+	std::string AccessKeyId = GetEnvVariable("AWS_ACCESS_KEY_ID").value_or("");
 	if (AccessKeyId.empty())
 	{
 		m_CredentialProvider = Ref<ImdsCredentialProvider>(new ImdsCredentialProvider({}));
@@ -2330,8 +2330,8 @@ S3Hydration::S3Hydration(const Configuration& Config) : HydrationBase(Config)
 	else
 	{
 		m_Credentials.AccessKeyId	  = std::move(AccessKeyId);
-		m_Credentials.SecretAccessKey = GetEnvVariable("AWS_SECRET_ACCESS_KEY");
-		m_Credentials.SessionToken	  = GetEnvVariable("AWS_SESSION_TOKEN");
+		m_Credentials.SecretAccessKey = GetEnvVariable("AWS_SECRET_ACCESS_KEY").value_or("");
+		m_Credentials.SessionToken	  = GetEnvVariable("AWS_SESSION_TOKEN").value_or("");
 	}
 
 	m_DefaultMultipartChunkSize = Settings["chunksize"sv].AsUInt64(DefaultMultipartChunkSize);
diff --git a/src/zenserver/hub/zenhubserver.cpp b/src/zenserver/hub/zenhubserver.cpp
index 27c1c9fc4..303b1f1b2 100644
--- a/src/zenserver/hub/zenhubserver.cpp
+++ b/src/zenserver/hub/zenhubserver.cpp
@@ -811,7 +811,7 @@ ZenHubServer::InitializeConsulRegistration(const ZenHubServerConfig& ServerConfi
 
 	std::string ConsulAccessTokenEnvName =
 		ServerConfig.ConsulTokenEnv.empty() ? GetDefaultConsulTokenEnvVariableName() : ServerConfig.ConsulTokenEnv;
-	std::string ConsulAccessToken = GetEnvVariable(ConsulAccessTokenEnvName);
+	std::string ConsulAccessToken = GetEnvVariable(ConsulAccessTokenEnvName).value_or("");
 	if (ConsulAccessToken.empty())
 	{
 		if (!ServerConfig.ConsulTokenEnv.empty())
diff --git a/src/zenserver/storage/projectstore/httpprojectstore.cpp b/src/zenserver/storage/projectstore/httpprojectstore.cpp
index 9844d02f0..c40690d5f 100644
--- a/src/zenserver/storage/projectstore/httpprojectstore.cpp
+++ b/src/zenserver/storage/projectstore/httpprojectstore.cpp
@@ -339,7 +339,7 @@ namespace {
 				std::string_view AccessTokenEnvVariable = Cloud["access-token-env"].AsString();
 				if (!AccessTokenEnvVariable.empty())
 				{
-					AccessToken = GetEnvVariable(AccessTokenEnvVariable);
+					AccessToken = GetEnvVariable(AccessTokenEnvVariable).value_or("");
 				}
 			}
 
@@ -474,7 +474,7 @@ namespace {
 				std::string_view AccessTokenEnvVariable = Builds["access-token-env"].AsString();
 				if (!AccessTokenEnvVariable.empty())
 				{
-					AccessToken = GetEnvVariable(AccessTokenEnvVariable);
+					AccessToken = GetEnvVariable(AccessTokenEnvVariable).value_or("");
 				}
 			}