feat: add endpoint and CLI for users to view their own OIDC claims (#23053)

- Adds a new API endpoint `GET /api/v2/users/oidc-claims` that returns
only the **merged claims** (not the separate id_token/userinfo
breakdown). Scoped exclusively to the authenticated user's own identity
— no user parameter, so users cannot view each other's claims.
- Adds a new CLI command:** `coder users oidc-claims` that hits the
above endpoint.
- The existing owner-only debug endpoint is preserved unchanged for
admins who need the full claim breakdown.


> 🤖 This PR was created with the help of Coder Agents, and will be
reviewed by my human. 🧑‍💻

cli/testdata/coder_users_--help.golden

@@ -8,16 +8,17 @@ USAGE:
   Aliases: user
 
 SUBCOMMANDS:
-    activate      Update a user's status to 'active'. Active users can fully
-                  interact with the platform
-    create        Create a new user.
-    delete        Delete a user by username or user_id.
-    edit-roles    Edit a user's roles by username or id
-    list          Prints the list of users.
-    show          Show a single user. Use 'me' to indicate the currently
-                  authenticated user.
-    suspend       Update a user's status to 'suspended'. A suspended user cannot
-                  log into the platform
+    activate       Update a user's status to 'active'. Active users can fully
+                   interact with the platform
+    create         Create a new user.
+    delete         Delete a user by username or user_id.
+    edit-roles     Edit a user's roles by username or id
+    list           Prints the list of users.
+    oidc-claims    Display the OIDC claims for the authenticated user.
+    show           Show a single user. Use 'me' to indicate the currently
+                   authenticated user.
+    suspend        Update a user's status to 'suspended'. A suspended user
+                   cannot log into the platform
 
 ———
 Run `coder --help` for a list of global options.