Guarantee FakeHttpMessageHandler disposal in AgentBlueprintServiceTests

[Copilot]

Explicit handler.Dispose() calls at the end of test bodies are skipped when assertions throw, leaking the handler and any queued HttpResponseMessage objects.

Changes

• 3 tests using CreateServiceWithFakeHandler() — wrap test body in using (handler) { ... } (tuple destructuring prevents using var on the declaration)
• 1 test with direct handler creation — replace var handler = new FakeHttpMessageHandler() + trailing Dispose() with using var handler = ...

// Before — disposal skipped if assertion throws
var (service, handler) = CreateServiceWithFakeHandler();
// ... assertions ...
handler.Dispose();

// After — disposal guaranteed
var (service, handler) = CreateServiceWithFakeHandler();
using (handler)
{
    // ... assertions ...
}

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