feat(stovepipe): add GetValidation protobuf contract

Summary:
Intent:
- Expose the current validation for a commit's authoritative Stovepipe request.
- Preserve result presence and bounded project enumeration in the public contract.

Changes:
- Add GetValidation request, response, and project validation messages.
- Reuse the shared validation result shape and generate gRPC and YARPC bindings.
- Clarify authoritative URI selection and pagination stability in the RFC.

This PR builds on #638, which introduces the request-history contract and shared validation result.

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Author: mnoah1

api/stovepipe/proto/stovepipe.proto

@@ -140,6 +140,50 @@ message GetRequestHistoryByURIResponse {
     string next_page_token = 2;
 }
 
+// GetValidationRequest selects the authoritative validation for an exact commit URI.
+message GetValidationRequest {
+    // Logical queue containing the request.
+    string queue = 1;
+    // Exact VCS-agnostic commit URI whose authoritative request is selected.
+    string change_uri = 2;
+    // Optional exact project scope. When absent, one page of planned projects is returned.
+    optional string project = 3;
+    // Maximum projects to return. Zero selects the server default.
+    int32 page_size = 4;
+    // Opaque continuation token. Empty selects the first page.
+    string page_token = 5;
+}
+
+// ProjectValidation is the recorded validation for one planned project.
+message ProjectValidation {
+    // Stable project identifier.
+    string project = 1;
+    // Immutable verdict for the project. Unset until its fact is recorded.
+    ValidationResult result = 2;
+}
+
+// GetValidationResponse contains the authoritative request's current validation projection.
+message GetValidationResponse {
+    // Globally unique identifier of the authoritative request.
+    string request_id = 1;
+    // Logical queue containing the request.
+    string queue = 2;
+    // VCS-agnostic commit URI validated by the request.
+    string change_uri = 3;
+    // Baseline URI for incremental validation. Empty for a full build.
+    string base_uri = 4;
+    // Stable public lifecycle state of the request.
+    string request_state = 5;
+    // Immutable whole-repository verdict. Unset until its fact is recorded.
+    ValidationResult repository_result = 6;
+    // Whether every planned project has one durable result and completion is recorded.
+    bool project_results_complete = 7;
+    // Planned projects in stable project order, with any recorded verdicts.
+    repeated ProjectValidation projects = 8;
+    // Opaque continuation token. Empty on the final page.
+    string next_page_token = 9;
+}
+
 // Stovepipe provides the Stovepipe API.
 service Stovepipe {
     // Ping returns a response indicating the service is alive
@@ -151,4 +195,6 @@ service Stovepipe {
     rpc GetRequestHistoryByRequestID(GetRequestHistoryByRequestIDRequest) returns (GetRequestHistoryByRequestIDResponse) {}
     // GetRequestHistoryByURI returns retained history for an exact commit URI's authoritative request.
     rpc GetRequestHistoryByURI(GetRequestHistoryByURIRequest) returns (GetRequestHistoryByURIResponse) {}
+    // GetValidation returns current validation for an exact commit URI's authoritative request.
+    rpc GetValidation(GetValidationRequest) returns (GetValidationResponse) {}
 }