Add HEX PM organization token support Pivate Registry on CI and update specs

[kbukum1]

What

Fixes #11760 - Migrate Hex integration tests from external self-hosted registry to team-controlled Hex.pm organization.

Why

The integration specs relied on dependabot-private.fly.dev, an external self-hosted registry that was frequently unavailable (503 errors), requiring complex mocking workarounds and causing flaky tests.

This PR migrates to our Hex.pm organization infrastructure for more reliable testing.

Approach: Real integration tests for hex_organization credentials (Hex.pm hosted), mocked tests for hex_repository credentials (self-hosted registries).

How

• Credential migration: Changed from hex_repository to hex_organization credential type
• Test fixtures: Updated to use test packages from dependabot organization on Hex.pm
• Skip logic: Tests gracefully skip without token, following Composer pattern
• CI integration: Added HEX_PM_ORGANIZATION_TOKEN environment variable to workflow
• Documentation: Created setup guide for contributors

Key Changes

• Integration specs - Migrated to hex_organization credentials with graceful skip logic
• Test fixtures - Updated to use dependabot organization packages and lockfile format
• CI workflow - Added HEX_PM_ORGANIZATION_TOKEN environment variable
• Documentation - New contributor setup guide

Testing

With token (CI after secret configured):

bin/docker-dev-shell hex
rspec spec/dependabot/hex/file_updater/lockfile_updater_spec.rb:440
rspec spec/dependabot/hex/update_checker_spec.rb:467

Without token (tests skip gracefully):

unset HEX_PM_ORGANIZATION_TOKEN
rspec spec/dependabot/hex/update_checker_spec.rb:437
# Output: "skipped because env var HEX_PM_ORGANIZATION_TOKEN is not set"