(PTFE-3084) Improve tests coverage

[matthiasL-scality]

This pull request introduces a comprehensive and robust end-to-end test suite for the artifacts service, covering authentication, health checks, build copying, build listings, and build discovery endpoints. It establishes reusable test fixtures and helpers for efficient setup and teardown of S3 buckets, HTTP sessions with various authentication levels, and common test operations. Shared constants are centralized for consistency across tests. The suite ensures correctness and security of key API endpoints and behaviors.

Test infrastructure and fixtures:

• Major refactor of conftest.py to provide session-scoped and function-scoped pytest fixtures for S3 client setup, bucket management, HTTP sessions with different authentication levels, and common test operations (uploading files, finishing builds). Includes robust S3 health check and cleanup logic.
• Shared constants for bucket names and canonical build names are defined in a new constants.py file, promoting reuse and consistency across the test suite.

Authentication and authorization coverage:

• Adds test_auth.py to verify access control: tests that authenticated users, restricted users, bots, failing users, and unauthenticated requests are all handled correctly with appropriate HTTP status codes for uploads, downloads, and metadata operations.

Endpoint-specific tests:

• Introduces test_healthcheck.py to validate that health check and nginx status endpoints respond as expected, including method restrictions and connection reporting.
• Implements test_copy.py to exercise the build copy endpoint, checking correct behavior for missing status files, duplicate copies, and cache/ingress scenarios.
• Adds test_listing.py to verify build and root listings, including pagination, format variations, and aggregation across buckets.
• Provides test_find_build.py to test "find latest build" endpoints (/last_success/, /last_failure/, /latest/), ensuring correct redirect and error behaviors for various build states and prefixes.

[bertranddemiddelaer]

LGTM