fix!: annotating some fields as REQUIRED

These fields were actually always required by the backend, so annotation just documents status quo. I believe this change will not require major version bump for any language.

PiperOrigin-RevId: 429093810

google/spanner/admin/instance/v1/spanner_instance_admin.proto

@@ -350,19 +350,22 @@ message Instance {
   // after the instance is created. Values are of the form
   // `projects/<project>/instances/[a-z][-a-z0-9]*[a-z0-9]`. The final
   // segment of the name must be between 2 and 64 characters in length.
-  string name = 1;
+  string name = 1 [(google.api.field_behavior) = REQUIRED];
 
   // Required. The name of the instance's configuration. Values are of the form
   // `projects/<project>/instanceConfigs/<configuration>`. See
   // also [InstanceConfig][google.spanner.admin.instance.v1.InstanceConfig] and
   // [ListInstanceConfigs][google.spanner.admin.instance.v1.InstanceAdmin.ListInstanceConfigs].
-  string config = 2 [(google.api.resource_reference) = {
-                       type: "spanner.googleapis.com/InstanceConfig"
-                     }];
+  string config = 2 [
+    (google.api.field_behavior) = REQUIRED,
+    (google.api.resource_reference) = {
+      type: "spanner.googleapis.com/InstanceConfig"
+    }
+  ];
 
   // Required. The descriptive name for this instance as it appears in UIs.
   // Must be unique per project and between 4 and 30 characters in length.
-  string display_name = 3;
+  string display_name = 3 [(google.api.field_behavior) = REQUIRED];
 
   // Required. The number of nodes allocated to this instance. This may be zero
   // in API responses for instances that are not yet in state `READY`.

google/spanner/v1/mutation.proto

@@ -17,6 +17,7 @@ syntax = "proto3";
 package google.spanner.v1;
 
 import "google/api/annotations.proto";
+import "google/api/field_behavior.proto";
 import "google/protobuf/struct.proto";
 import "google/spanner/v1/keys.proto";
 
@@ -36,7 +37,7 @@ message Mutation {
   // [replace][google.spanner.v1.Mutation.replace] operations.
   message Write {
     // Required. The table whose rows will be written.
-    string table = 1;
+    string table = 1 [(google.api.field_behavior) = REQUIRED];
 
     // The names of the columns in [table][google.spanner.v1.Mutation.Write.table] to be written.
     //
@@ -59,7 +60,7 @@ message Mutation {
   // Arguments to [delete][google.spanner.v1.Mutation.delete] operations.
   message Delete {
     // Required. The table whose rows will be deleted.
-    string table = 1;
+    string table = 1 [(google.api.field_behavior) = REQUIRED];
 
     // Required. The primary keys of the rows within [table][google.spanner.v1.Mutation.Delete.table] to delete.  The
     // primary keys must be specified in the order in which they appear in the
@@ -67,7 +68,7 @@ message Mutation {
     // statement used to create the table).
     // Delete is idempotent. The transaction will succeed even if some or all
     // rows do not exist.
-    KeySet key_set = 2;
+    KeySet key_set = 2 [(google.api.field_behavior) = REQUIRED];
   }
 
   // Required. The operation to perform.

google/spanner/v1/spanner.proto

@@ -300,8 +300,8 @@ message CreateSessionRequest {
     }
   ];
 
-  // The session to create.
-  Session session = 2;
+  // Required. The session to create.
+  Session session = 2 [(google.api.field_behavior) = REQUIRED];
 }
 
 // The request for [BatchCreateSessions][google.spanner.v1.Spanner.BatchCreateSessions].
@@ -647,7 +647,7 @@ message ExecuteBatchDmlRequest {
   // A single DML statement.
   message Statement {
     // Required. The DML string.
-    string sql = 1;
+    string sql = 1 [(google.api.field_behavior) = REQUIRED];
 
     // Parameter names and values that bind to placeholders in the DML string.
     //