feat(client): add proxy object for request.agent for compatibility

[codders]

In node environments, the code in node/agent.js creates a proxy object around Agent so that it can be instantiated without using the new keyword. The equivalent code for the browser version (in client.js) defines request.agent as () => new Agent().

In supertest, the TestAgent attempts to subclass Agent with the call Object.setPrototypeOf(TestAgent.prototype, Agent.prototype);. This works in the node environment but not in the browser version, because in the browser version () => new Agent() is not an object and has an undefined prototype, leading to the error message Object prototype may only be an Object or null: undefined.

Wrap request.agent in a proxy object for the client/browser case so that supertest can also use the library in browser contexts without error.

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
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• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.

[codders]

Fixes #1840