feat: Implement additive options merging for cascade bind calls

[davidmigloz]

Relates to #481

Previously, if you had multiple bind calls in cascade, the options from the previous bind call were overwritten by the following bind call. This PR introduces a change where options provided in successive bind calls are now merged additively.

Non-null fields in the options of a subsequent bind call will override those in the existing options, while preserving any other existing options that were not explicitly overridden.

Example:

final chatModel = ChatOpenAI(
  apiKey: openAiApiKey,
  defaultOptions: const ChatOpenAIOptions(
    model: 'gpt-4o',
    temperature: 0,
  ),
);

final chatModelWithTools = chatModel.bind(
  const ChatOpenAIOptions(
    tools: [getCurrentWeatherTool, jokeTool],
  ),
);

final res1 = await chatModelWithTools.invoke(
  PromptValue.string(
    'Tell me the weather in Barcelona, Spain and a joke about bears',
  ),
);
print(res1.output.toolCalls);
// [AIChatMessageToolCall{
//   id: call_b7WEVe4FsnNMPdxoGnwnYmE1,
//   name: get_current_weather,
//   argumentsRaw: {"location": "Barcelona, Spain"},
//   arguments: {location: Barcelona, Spain},
// }, AIChatMessageToolCall{
//   id: call_TIhoSK8pfAi7KaOd0rCDvj3C,
//   name: joke,
//   argumentsRaw: {"setup": "Why don't bears wear shoes?", "punchline": "Because they have bear feet!"},
//   arguments: {setup: Why don't bears wear shoes?, punchline: Because they have bear feet!},
// }]

final chatModelForceWeatherTool = chatModelWithTools.bind(
  ChatOpenAIOptions(
    toolChoice: ChatToolChoice.forced(name: getCurrentWeatherTool.name),
  ),
);

final res2 = await chatModelForceWeatherTool.invoke(
  PromptValue.string(
    'Tell me the weather in Barcelona, Spain and a joke about bears',
  ),
);
print(res2.output.toolCalls);
// [AIChatMessageToolCall{
//   id: call_halcHZRWDLgs82MJW3WkUitg,
//   name: get_current_weather,
//   argumentsRaw: {"location":"Barcelona, Spain","unit":"celsius"},
//   arguments: {location: Barcelona, Spain, unit: celsius},
// }]

final chatModelForceJokeTool = chatModelWithTools.bind(
  ChatOpenAIOptions(
    toolChoice: ChatToolChoice.forced(name: jokeTool.name),
  ),
);

final res3 = await chatModelForceJokeTool.invoke(
  PromptValue.string(
    'Tell me the weather in Barcelona, Spain and a joke about bears',
  ),
);
print(res3.output.toolCalls);
// [AIChatMessageToolCall{
//   id: call_CnijbH1AQNnSh2XEG2JbKcsZ,
//   name: joke,
//   argumentsRaw: {"punchline":"Because they always bring their own bear-ings!"},
//   arguments: {punchline: Because they always bring their own bear-ings!},
// }]