feat(DEBT-67): add opt-in single-line error display

[RuellePaul]

What

Opt-in simplified error display: SDK errors become a single concise line instead of a full Python traceback + raw GraphQL payload.

Enable it with the KILI_SDK_SIMPLIFY_ERROR_LOGS environment variable (true/1/yes), or with "simplify_error_logs": true in kili-sdk-config.json. Default behavior is unchanged when the flag is not set.

After (flag enabled)

$ python scripts/check_project_status.py
🔍 Vérification du projet cme2rmsjdg0k4an0w4j0iggq3
Project with id cme2rmsjdg0k4an0w4j0iggq3 not found

$ python scripts/add_or_set_metadata.py   # invalid API key
UserWarning: Client domain api is still a work in progress. Method names and return type will evolve.
Invalid API key `**gerg`

How

• New kili/core/simplified_errors.py:
  • flag resolution (env var takes precedence over the config file)
  • simplify_graphql_error_message() strips [errorCode] prefixes, the "This can be due to:" boilerplate and backend stack-trace lines
  • sys.excepthook renders uncaught kili.* exceptions as a single line — non-Kili exceptions keep their full traceback
  • IPython/Jupyter custom exception handler for notebooks
  • SDK warnings formatted on a single line
• GraphQLError / AuthenticationFailed build short messages when the flag is enabled (e.g. Invalid API key `**gerg` ); also fixes an unbalanced quote in the batched-error message
• Hooks are installed at kili import time (env var) and at client init (config file), including the domain client before its early warning
• Docs: new section in docs/configuration.md, env var reference table, kili-sdk-config.example.json

Tests

• 27 new unit tests in tests/unit/core/test_simplified_errors.py; full unit suite green; ruff / pylint 10.00 / pyright clean
• Verified end-to-end against a local backend: invalid API key and unknown project scenarios produce the expected single-line output; legacy output is byte-identical without the flag

DEBT-67 — companion of the label-backend-v2 MR Draft: fix(DEBT-67): remove stack traces from client-facing GraphQL error messages (kili-technology/kili!14274).