feat: mark default superuser identities in AuthenticationResult context

[cecemei]

Description

Adds a superuser context key to basic-security extension's default superuser identities.

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[gianm]

The new key doesn't seem to serve a purpose, since as far as I can tell it isn't read by anything. It also seems orthogonal to the permission system in general, which is potentially confusing.

[FrankChen021]

🟢 Approval recommended

No actionable issues found. The marker is applied across HTTP and JDBC basic authentication, escalated authentication results, and default-user initialization without changing existing authorization behavior. Reviewed 5 of 5 changed files, including all production changes, the added unit tests, and relevant AuthenticationResult consumers, validators, role providers, and escalator call sites. Static review only; no build or test commands were run.

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This is an automated review by Codex GPT-5.6-Luna(max)