[feature request][bug] Expose principal_id in Principals REST API / Improve OIDC Mapping Behavior

### Is your feature request related to a problem? Please describe.

The current Principals REST API does not return principal_id, but external OIDC providers are expected to supply:

- principal_id
- principal_name
- principal_roles

Because **principal_id** is not exposed via the API, external identity providers cannot reliably provide it. This forces users to choose between:

- Accessing the metastor and manually retrieving IDs from the internal entities table, or

- Using the **undocumented workaround** of returning `principal_id = 0`, which causes Polaris to fall back to name-based mapping and ignore ID-based mapping entirely.
https://github.com/apache/polaris/blob/3c5dbaf2bc93bb2861b5fb0407ffc1597e48bdd2/runtime/service/src/main/java/org/apache/polaris/service/auth/DefaultAuthenticator.java#L116

This silent behavior is not discoverable by users unless they read the source code.

### Describe the solution you'd like

#### Expose principal_id in the Principals REST API

Return principal_id in responses so external OIDC providers can supply accurate mapping data.

#### Document the principal_id = 0 Fallback Behavior

If exposing internal IDs is not acceptable, the documentation should explicitly state:
Supplying `principal_id = 0` disables ID-based matching and triggers name-based mapping.

### Describe alternatives you've considered

#### Remove ID-Based Mapping Logic Entirely

Standardize principal mapping to rely solely on names (and optionally roles), avoiding IDs altogether.

### Additional context

- OIDC providers cannot determine or supply the correct internal principal_id.
- Users unknowingly depend on an undocumented behavior (`principal_id = 0`) to make authentication work.
- The current mix of ID-based and name-based logic is ambiguous and confusing.
- The user experience degrades because the expected mapping logic is not transparent.

Start with runtime/service/src/main/java/org/apache/polaris/service/auth/DefaultAuthenticator.java at the linked mapping logic, then trace the Principals REST API response path. Clarify whether the intended outcome is exposing principal_id, documenting the principal_id = 0 fallback, or changing the mapping model. Done means the chosen behavior is implemented or documented and the ambiguity is removed.

Written by the indexing model from the issue text.