feat(authz): defer to request auth as decision/entitlements entity (#2789)

### Proposed Changes

* ADR for 4th entity identifier proto oneof value
* proto for 4th entity identifier oneof value (a boolean to pull entity
token from request auth)

### Checklist

- [ ] I have added or updated unit tests
- [ ] I have added or updated integration tests (if appropriate)
- [ ] I have added or updated documentation

### Testing Instructions

Author: jakedoublev

adr/decisions/2025-10-07-authz-entity-request-auth.md

@@ -0,0 +1,58 @@
+---
+status: 'proposed'
+date: '2025-10-07'
+tags:
+ - authorization
+driver: '@jakedoublev'
+consulted:
+  - '@jrschumacher'
+  - '@c-r33d'
+---
+# Allow Authz GetDecision/GetEntitlements requests to derive entity from request authorization token JWT
+
+## Context and Problem Statement
+
+Today, Authorization endpoints require entity specification on the request body, including one of a
+`Token` option, a `RegisteredResourceFQN` option, or an `EntityChain` option.
+
+In our SDKs, we prefer client-side interceptors to populate auth state within authenticated requests.
+
+When adding SDK support for a TDF `.Obligations()` method, it was realized we would need to add auth logic inside
+each SDK method itself (not their interceptors) to retrieve a valid access token and set it to a `GetDecision` request
+body as a `Token` entity. We had not exposed the ability serverside to check the request authorization token _as_ the entity
+in a decision/entitlements request, and would need to do more work clientside as a result.
+
+## Decision Drivers
+
+* Reduced SDK logic:
+    - avoids roundtripping for an access token from the idP twice (once to auth the request, once to place in the body)
+    - avoids complexity in the interceptors (check if we've already retrieved a token first, etc.)
+* Improved DX/UX:
+    - effectively GetMyEntitlements, GetMyDecision
+* Security:
+    - fewer access tokens retrieved by an SDK instance
+    - no changes to casbin RBAC route authz policy
+    - any users permitted to make GetDecision/GetEntitlement queries will only get to check their own tokens more easily (inherently more secure than ability to check someone else's)
+    - tokens have already been available to in-process services, and this work builds upon that feature
+    - no security footguns when decisioning/entitling as the request auth usage is _explicit_ not _implicit_
+
+Note: platform security controls safeguarding request access tokens as platform-level resources (should an IPC PEP
+be allowed to touch an access token) is considered out of scope
+
+## Considered Options
+
+* **Option 1**: Get an Access Token outside the interceptor from the OAuth token source for request body entity _and_ another within the interceptor (two tokens retrieved)
+* **Option 2**: Get an Access Token outside the interceptor from the OAuth token source for request body entity _and_ reuse within interceptor (one token retrieved)
+* **Option 3**: Improve Authz endpoints to allow explicitly using the auth header token as the entity
+
+## Decision Outcome
+
+Chosen option: **Option 3**: Improve Authz endpoints to allow explicitly using the auth header token as the entity
+
+### Consequences
+
+* 🟩 **Good**, because better API UX/DX for use cases where a PEP auth == user auth
+* 🟩 **Good**, because keeps token handling logic in SDKs centralized
+* 🟩 **Good**, because simplified Obligations decision check flow
+* 🟩 **Good**, because explicit flag on request avoids footguns in super-privileged PEPs
+* 🟨 **Neutral**, because more complexity in a oneof proto, but there are 4 types of entities/entity sources