Concurrent requests lead to duplicated responses

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Environment

- FlareSolverr version: 3.4.6
- Last working FlareSolverr version:
- Operating system: Ubuntu 24.04.2 LTS
- Are you using Docker: yes
- FlareSolverr User-Agent (see log traces or / endpoint): Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/142.0.0.0 Safari/537.36
- Are you using a VPN: no
- Are you using a Proxy: no
- Are you using Captcha Solver: no
- If using captcha solver, which one:
- URL to test this issue:

Description

When a FlareSolverr session reuses a single WebDriver instance, concurrent requests that specify the same session can overlap and interfere with each other. If request A is still being processed and request B starts using the same session/WebDriver, B can receive A's response.

You can use these steps easily reproduce the issue:

1. In terminal 1, run the following curl command:

   curl -v -X POST 'http://localhost:8191/v1' -H 'Content-Type: application/json' -d '{"cmd":"request.get","url":"https://httpbin.org/get?v=1","session":"test", "waitInSeconds": 5}'
   

2. In terminal 2, run the curl command with a different URL simultaneously:

   curl -v -X POST 'http://localhost:8191/v1' -H 'Content-Type: application/json' -d '{"cmd":"request.get","url":"https://httpbin.org/get?v=2","session":"test"}'
   

The address requested in the WebDriver session will be overritten by the second request, and both requests will receive the response for the second URL (https://httpbin.org/get?v=2).

It is not necessarily using a waitInSeconds parameter that causes the issue, any simultanesous requests may cause the problem leading to wrong responses.

I think the best solution is to use a lock to ensure that only one request can use the WebDriver session.

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