Full session transcript not being sent models

[nathanborror]

Full conversation transcript isn't being passed to subsequent model requests. Adjusting the conversationContext test reveals this:

@Test func conversationContext() async throws {
    let session = LanguageModelSession(model: model)
    
    let firstResponse = try await session.respond(to: "My favorite color is blue")
    #expect(!firstResponse.content.isEmpty)

    let secondResponse = try await session.respond(to: "What did I just tell you?")
    #expect(secondResponse.content.contains("color"))
}

Seems to be true for OpenAI, Anthropic and Gemini.

[nathanborror]

Working on a PR for this.