Add integration tests for migration categories and execution

- Implemented MigrationCategoryTests to validate migration categorization for startup, release, seed, and data migrations.
- Added tests for edge cases, including null, empty, and whitespace migration names.
- Created StartupMigrationHostTests to verify the behavior of the migration host with real PostgreSQL instances using Testcontainers.
- Included tests for migration execution, schema creation, and handling of pending release migrations.
- Added SQL migration files for testing: creating a test table, adding a column, a release migration, and seeding data.

src/Cli/StellaOps.Cli/Services/Models/PolicyWorkspaceModels.cs

@@ -519,17 +519,8 @@ internal sealed class PolicyDiagnostic
     [JsonPropertyName("severity")]
     public string Severity { get; init; } = "error";
 
-    [JsonPropertyName("line")]
-    public int? Line { get; init; }
-
-    [JsonPropertyName("column")]
-    public int? Column { get; init; }
-
-    [JsonPropertyName("span")]
-    public string? Span { get; init; }
-
-    [JsonPropertyName("suggestion")]
-    public string? Suggestion { get; init; }
+    [JsonPropertyName("path")]
+    public string? Path { get; init; }
 }
 
 // CLI-POLICY-27-002: Policy submission/review workflow models