Standardise poetry virtual env config

What does this merge request do and why?

Standardise poetry virtual env config

• Create the virtualenv inside the project’s root directory.
• All needed files are copied into it instead of symlinked.

How to set up and validate locally

1. Check out to this merge request's branch.
2. Verify that the config is setup correctly.

   $ poetry config --list -vvv
   Loading configuration file /Users/tanle/Library/Application Support/pypoetry/config.toml
   Loading configuration file /Users/tanle/code/gitlab/ai-assist-api/poetry.toml
   cache-dir = "/Users/tanle/Library/Caches/pypoetry"
   --snip--
   virtualenvs.create = true
   virtualenvs.in-project = true
   virtualenvs.options.always-copy = true
   --snip--

3. Recreatd the current virtual env.

   $ rm -rf .venv
   $ poetry install
   Creating virtualenv ai-gateway in /Users/tanle/code/gitlab/ai-assist-api/.venv
   --snip--

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.