extend_content does not receive the security context in cube.py

[igorlukanin]

Describe the bug
extend_context configuration parameter allows to extend the security context, as described in the docs and as expected, when provided via the cube.js file.

However, when provided via the cube.py file, the res (first) parameter to extend_context does not contain securityContext, so there's no way to extend it, just provide a new one from scratch.

To Reproduce
Steps to reproduce the behavior:

1. Use the following cube.py file:

from cube import config
 
@config('check_auth')
def check_auth(ctx: dict, token: str) -> None:
  new_ctx = {
    'security_context': {
      'user_id': 42
    }
  }

  return new_ctx
 
@config('extend_context')
def extend_context(req: dict) -> dict:
  req.setdefault('securityContext', {})
  print(req['securityContext'])

  return req

@config('context_to_app_id')
def context_to_app_id(ctx: dict) -> dict:
  return f"CUBE_APP_{str(ctx['securityContext'])}"

2. Run a query
3. See only empty objects/dictionaries ({}) printed in the console
4. Alternatively, try the following, equivalent cube.js file

module.exports = {
  checkAuth: (ctx, token) => {
    let new_ctx = {
      'security_context': {
        'user_id': 42
      }
    }

    return new_ctx
  },

  extendContext: (req) => {
    console.log(req['securityContext'])

    return req
  },

  contextToAppId: (ctx) => {
    return `CUBE_APP_${JSON.stringify(ctx['securityContext'])}`
  }
}

5. See extended security context printed in the console

Expected behavior
extend_context works as described in docs in both cube.js and cube.py files.

Version:
v0.36.3

Additional context
Discovered via Slack: https://cube-js.slack.com/archives/C04NYBJP7RQ/p1727431492374679?thread_ts=1727361286.150869&cid=C04NYBJP7RQ