auth: return linked wallet on Google exchange

grid_api/routers/AGENTS.md

@@ -47,7 +47,9 @@ transport, accounts, stats, health/metrics.
   wallet, then verified email only when it is the sole identity); supplemental
   or unverified email must never join accounts.
   Native service/app exchange lives at `/v1/auth/service/exchange`; Google ID
-  tokens are verified at `/v1/auth/google/exchange`; partner wallet proof uses
+  tokens are verified at `/v1/auth/google/exchange`, which also returns the
+  canonical account's primary verified wallet when one is linked; partner
+  wallet proof uses
   `/v1/auth/wallet/challenge` plus `/v1/auth/wallet/exchange`, bound to the
   service, its exact `siwe_domains`, the app subject, wallet, URI, Base chain,
   expiry, and one-use nonce. `/v1/auth/service/bind` binds an app subject after

grid_api/routers/accounts.py

@@ -943,7 +943,14 @@ async def exchange_google_identity(
         account_id=account_id,
         ref=service_auth.new_event_ref("exchange", service["service_id"]),
     )
-    return {"access_token": token, "token_type": "Bearer", "expires_in": 900, "account_id": str(account_id)}
+    wallets = await identities_svc.verified_wallet_addresses(account_id)
+    return {
+        "access_token": token,
+        "token_type": "Bearer",
+        "expires_in": 900,
+        "account_id": str(account_id),
+        "wallet": wallets[0] if wallets else None,
+    }
 
 
 @router.post("/v1/auth/service/bind")

grid_api/services/tests/test_native_service_auth.py

@@ -362,6 +362,18 @@ async def no_value(*_args, **_kwargs):
     ) == canonical_id
     assert await credits.get_balance(UUID(canonical_id)) == 20_000
 
+    repeated_google = await accounts_router.exchange_google_identity(
+        request,
+        accounts_router.GoogleExchangeForm(
+            id_token="google-proof-after-wallet-link",
+            app_subject=app_subject,
+        ),
+        apikey=key,
+        authorization=None,
+    )
+    assert repeated_google["account_id"] == canonical_id
+    assert repeated_google["wallet"] == wallet.address.lower()
+
 
 @pytest.mark.asyncio
 async def test_verified_wallet_account_and_balance_are_shared_across_products(