feat(auth): expose OAuth scope contract

[lkretvix]

What does this PR do?

Adds a machine-readable Pup auth scope contract for downstream consumers:

• Introduces pup auth scopes --output=json.
• Emits schema_version, the default pup auth login OAuth scopes, and source: "pup auth login" without requiring authentication or config loading.
• Reuses the same default-scope helper for auth login and the new contract so they cannot drift silently.
• Adds integration coverage for the JSON output and for rejecting non-JSON output.

Motivation

Bits needs to decide whether Pup is installed and, when available, request the same OAuth scope set Pup expects. A stable JSON command lets Bits feature-probe Pup directly instead of hard-coding or scraping CLI behavior.

Additional Notes

Validation performed:

• cargo test auth_scopes_json
• cargo test default_login_scopes_match_auth_scopes_contract
• cargo test auth::types::tests::
• cargo run -- auth scopes --output=json
• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test -- --test-threads=1

A parallel cargo test run exposed existing mock-base-url interference in five unrelated tests that attempted to call http://unused.local; each failed test passed when rerun individually, and the full suite passed with --test-threads=1.

Checklist

• The code change follows the project conventions (see CONTRIBUTING.md)
• Tests have been added/updated (if applicable)
• Documentation has been updated (if applicable)
• All CI checks pass
• Code coverage is maintained or improved

Related Issues

Part of the Pup Agent Integration plan.

[lkretvix]

remove the mention of bits here