Consolidate dual build validation paths (DeployableBuildFactory vs RequestsManager)

Background

PR #584 introduced IBuildServerClientFactory used by both DeployableBuildFactory and RequestsManager. These two paths both independently resolve the project, create build clients, and validate builds with divergent error handling.

Problem

• DeployableBuildFactory.CreateInstance() → IDeployableBuild.IsValid() validates synchronously
• RequestsManager.BuildServerDetailAsync() → validates asynchronously via ValidateBuildAsync()

Both call GetProject() independently and create separate client instances. Error handling differs.

Impact

Parallel paths will diverge in behaviour over time, creating subtle bugs where one accepts a build the other rejects.

Recommended Approach

Create a BuildValidationService that encapsulates project lookup + client creation + validation. Both DeployableBuildFactory and RequestsManager delegate to this service with consistent error handling.

Identified in multi-model review of PR #584 (Finding #10)

Start by reading DeployableBuildFactory.CreateInstance(), IDeployableBuild.IsValid(), RequestsManager.BuildServerDetailAsync(), and ValidateBuildAsync(), along with the IBuildServerClientFactory usage introduced in PR #584. Trace how each path resolves projects, creates clients, and handles errors. Done means both paths delegate project lookup, client creation, and validation to one service with consistent behavior.