Handle Octokit::UnprocessableEntity in GitHub solution syncer PR creation (#8609)

When there are no commits between the base and sync branches, GitHub
returns a 422 error. This can happen when the sync process determines
there are changes but the resulting commits don't differ from the base.
Handle this the same way as NotFound and Forbidden — as a permanent
failure that doesn't need retrying.

Closes #8596

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: iHiD

app/commands/user/github_solution_syncer/create_pull_request.rb

@@ -23,9 +23,9 @@ def call
         pr_title,
         pr_message
       )
-    rescue Octokit::NotFound, Octokit::Forbidden
-      # Repo may have been deleted/renamed, or the integration
-      # no longer has permission — nothing to sync to
+    rescue Octokit::NotFound, Octokit::Forbidden, Octokit::UnprocessableEntity
+      # Repo may have been deleted/renamed, the integration no longer
+      # has permission, or there are no commits between branches
       nil
     end
 

test/commands/user/github_solution_syncer/create_pull_request_test.rb

@@ -70,5 +70,33 @@ class CreatePullRequestTest < ActiveSupport::TestCase
       result = User::GithubSolutionSyncer::CreatePullRequest.(syncer, "title", "body", &block)
       assert_nil result
     end
+
+    test "returns nil when PR validation fails" do
+      syncer = create :user_github_solution_syncer
+
+      hex_8 = SecureRandom.hex(8)
+      SecureRandom.expects(:hex).with(8).returns(hex_8)
+
+      token = "fake-token-#{SecureRandom.uuid}"
+      GithubApp.expects(:generate_installation_token!).with(syncer.installation_id).returns(token)
+
+      client = mock
+      Octokit::Client.expects(:new).with(access_token: token).returns(client).at_least_once
+
+      base_branch = "main"
+      base_sha = "base-commit-sha"
+      new_branch = "exercism-sync/#{hex_8}"
+      client.expects(:repository).with(syncer.repo_full_name).returns(mock(default_branch: base_branch))
+      client.expects(:branch).with(syncer.repo_full_name, base_branch).returns(mock(commit: mock(sha: base_sha)))
+      client.expects(:create_ref).with(syncer.repo_full_name, "heads/#{new_branch}", base_sha)
+
+      block = ->(_, _) {}
+      block.expects(:call).with(new_branch, token).returns(true)
+
+      client.expects(:create_pull_request).raises(Octokit::UnprocessableEntity)
+
+      result = User::GithubSolutionSyncer::CreatePullRequest.(syncer, "title", "body", &block)
+      assert_nil result
+    end
   end
 end