Refactor to use IHttpClientFactory and implement IDisposable

[jmarbutt]

Problem

InngestClient owns a raw HttpClient, isn't IDisposable, and doesn't use IHttpClientFactory. This pattern has several issues:

1. Socket exhaustion - Creating multiple clients risks exhausting sockets
2. Configuration - Makes global configuration (timeouts, handlers, logging) harder
3. Testing - Harder to mock HTTP calls in tests
4. Best practices - Doesn't follow .NET HttpClient best practices

Location: Inngest/InngestClient.cs constructors

Suggested Solution

1. Use IHttpClientFactory for HttpClient management
2. Register a named/typed client in DI: services.AddHttpClient<InngestClient>()
3. Implement IDisposable if the client owns any unmanaged resources
4. Allow configuration via IHttpClientBuilder for policies, handlers, etc.

Benefits

• Proper connection pooling and lifecycle management
• Easier to add Polly policies for retries/circuit breakers
• Better testability with MockHttpMessageHandler
• Follows .NET best practices

Priority

Medium - important for production reliability.