Remove sync-over-async pattern from GitHub Actions integration

Background

PR #584 introduced GitHub Actions support. Several code paths use .GetAwaiter().GetResult() to block on async HTTP calls, risking thread-pool starvation and deadlocks.

Affected Code

1. GitHubActionsBuildServerClient.GetBuildDefinitions() — blocks on HttpClient.GetAsync() in a foreach loop
2. GitHubDeployableBuild.IsValid() — blocks on ValidateBuildAsync()
3. RequestsManager.RequestDetail() — blocks on BuildServerDetailAsync()

Why Deferred

Same pattern exists in pre-existing Azure DevOps code paths. Fixing requires making IBuildServerClient.GetBuildDefinitions, IDeployableBuild.IsValid, and RequestsManager.RequestDetail fully async, plus updating all callers.

Recommended Approach

1. IBuildServerClient.GetBuildDefinitions → GetBuildDefinitionsAsync returning Task<>
2. IDeployableBuild.IsValid → IsValidAsync returning Task<bool>
3. RequestsManager.RequestDetail → RequestDetailAsync
4. Update all controller/service callers to await

Identified in multi-model review of PR #584 (Finding #7)

Start by tracing the implementations and callers of IBuildServerClient.GetBuildDefinitions, IDeployableBuild.IsValid, and RequestsManager.RequestDetail, including the GitHub Actions and pre-existing Azure DevOps paths. The work is done when these interfaces and their callers are fully async, return the specified Task-based results, and no longer block on the listed async operations.