fix(cli): warn when auth validation falls back to API

[puneetdixit200]

Description

Log a warning when local CLI access-token validation is skipped because the auth server cannot be reached. This preserves the existing offline/flaky-network fallback while making it explicit that token freshness was not proven locally and the API will make the final authorization decision.

Related Issue

Fixes #1208

Motivation and Context

The existing fallback is useful for availability, but silent success makes the path look like a completed local validation. A warning keeps the behavior unchanged while surfacing the caveat from the issue.

How Has This Been Tested?

Red before implementation:

uv run --group dev python -m pytest -q tests/cli/test_cli_auth.py::TestAuthMethods::test_validate_access_token_network_error_returns_true

Green after implementation:

uv run --group dev python -m pytest -q tests/cli/test_cli_auth.py::TestAuthMethods::test_validate_access_token_network_error_returns_true
uv run --group dev python -m pytest -q tests/cli/test_cli_auth.py tests/cli/test_cli.py tests/cli/test_cli_main.py
uv run --group dev black --check codecarbon/cli/auth.py tests/cli/test_cli_auth.py
uv run --group dev ruff check codecarbon/cli/auth.py tests/cli/test_cli_auth.py
git diff HEAD^ --check

Screenshots (if appropriate):

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

AI Usage Disclosure

OpenAI GPT-5 generated the small code/test change and PR text under explicit human-directed issue-fix constraints. I reviewed the touched auth path, preserved existing fallback behavior, and verified the change with the commands listed above.

• 🟥 AI-vibecoded: You cannot explain the logic. Car analogy : the car drive by itself, you are outside it and just tell it where to go.
• 🟠 AI-generated: Car analogy : the car drive by itself, you are inside and give instructions.
• ⭐ AI-assisted. Car analogy : you drive the car, AI help you find your way.
• ♻️ No AI used. Car analogy : you drive the car.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the docs/how-to/contributing.md document.
• I have added tests to cover my changes.
• All new and existing tests passed.