[AZ-577] [AZ-578] [AZ-579] [AZ-580] Implement E2E test batch 2

Adds 26 blackbox tests (FT-P-01..18, FT-N-01..08) covering full AC
matrices for Vehicles/Missions/Waypoints/Health/Errors. Three
spec-vs-code carry-forwards documented in batch_02_report.md and
pinned with [Trait("carry_forward", ...)].

Shared scaffolding: ApiDtos.cs, AssertProblemEnvelopeAsync helper,
Seeds.cs, StubSchema.cs, CascadeF3/F4 fixtures, PostgresStopStart
fixture (gated by COMPOSE_RESTART_ENABLED). Removes the 4 placeholder
Sanity.cs files (now superseded). docker-compose.test.yml gains the
expected_results volume mount + FIXTURE_SQL_DIR for the consumer.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/Azaion.Missions.E2E.Tests/Helpers/HttpAssertions.cs

@@ -29,6 +29,42 @@ public static class HttpAssertions
         AssertNoStackLeak(body);
     }
 
+    /// <summary>
+    /// Asserts the {statusCode, message} envelope produced by
+    /// <c>ErrorHandlingMiddleware</c>. The envelope uses camelCase keys
+    /// because the middleware emits an anonymous object literal — see
+    /// _docs/02_document/components/06_http_conventions/description.md.
+    /// </summary>
+    public static async Task<ProblemDto> AssertProblemEnvelopeAsync(
+        HttpResponseMessage response,
+        HttpStatusCode expectedStatus)
+    {
+        await AssertStatusAsync(response, expectedStatus).ConfigureAwait(false);
+        var body = await response.Content.ReadFromJsonAsync<JsonElement>().ConfigureAwait(false);
+
+        Assert.True(body.TryGetProperty("statusCode", out var statusEl),
+            "problem envelope missing 'statusCode' property");
+        Assert.True(body.TryGetProperty("message", out var messageEl),
+            "problem envelope missing 'message' property");
+        Assert.Equal((int)expectedStatus, statusEl.GetInt32());
+        var message = messageEl.GetString();
+        Assert.False(string.IsNullOrEmpty(message),
+            "problem envelope 'message' must be non-empty");
+
+        AssertNoStackLeak(body);
+
+        // Reject any extra keys to pin the envelope contract — the spec says
+        // EXACTLY these two keys (results_report.md row 1.8 + AC-8.6).
+        var extraKeys = body.EnumerateObject()
+            .Select(p => p.Name)
+            .Where(n => n is not ("statusCode" or "message"))
+            .ToArray();
+        Assert.True(extraKeys.Length == 0,
+            $"problem envelope has unexpected extra keys: {string.Join(",", extraKeys)}");
+
+        return new ProblemDto(statusEl.GetInt32(), message!);
+    }
+
     public static void AssertNoStackLeak(JsonElement body)
     {
         // Walk the JSON DOM and fail if any key looks like it leaks server internals.