fix: never show one az account's subscriptions under another (#3)

With two az accounts, signing in to one could make the other account's group list the first one's subscriptions.

**Why:** az can't be told which user to get a token for, so each account is pinned with `--subscription`, using one of its subscriptions. The pin was chosen once at start. But az's profile has one entry per subscription, tagged with the account that signed in to it last. Signing in to another account that also sees the pinned subscription moves it there, and from then on the first account's token requests silently returned the other account's token. That's what happened here: the in-app sign-in as one account re-tagged the other account's default subscription.

**Fix:**
- The pin is looked up in az's profile for every new token (only when the in-memory token cache misses), so it follows az's current mapping. An account with no subscriptions left in az gets "sign in as …".
- A token az issues for **another of the signed-in users** (by its `upn`/`unique_name`/`preferred_username`/`email` claim) is refused with "az gave a token for X, not Y: sign in as Y again", which shows as the usual sign-in prompt. Only a match against another known user is refused, so tokens without those claims (service principals) or with unusual guest names aren't turned away.

Tests cover the subscription moving between users, a refused token, a user gone from az, and the claim matching. I also checked against two real az accounts (one signed in, one expired): the signed-in account lists its subscriptions, and the expired one shows the sign-in error.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>
Reviewed-on: #3

internal/provider/azure/accounts_test.go

@@ -2,6 +2,7 @@ package azure
 
 import (
 	"context"
+	"encoding/base64"
 	"errors"
 	"fmt"
 	"os"
@@ -11,6 +12,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/Azure/azure-sdk-for-go/sdk/azcore"
 	"github.com/Azure/azure-sdk-for-go/sdk/azcore/policy"
 )
 
@@ -67,7 +69,9 @@ func TestAccountLabels(t *testing.T) {
 
 func TestCLICredentialPinsAccount(t *testing.T) {
 	var calls [][]string
-	c := newCLICredential("s-alice-2", "t1")
+	c := newCLICredential("alice@contoso.com", "t1", func() ([]account, error) {
+		return []account{{user: "alice@contoso.com", tenant: "t1", anchor: "s-alice-2"}}, nil
+	})
 	c.run = func(_ context.Context, args ...string) ([]byte, error) {
 		calls = append(calls, args)
 		exp := time.Now().Add(time.Hour).Unix()
@@ -102,7 +106,7 @@ func TestCLICredentialPinsAccount(t *testing.T) {
 
 func TestCLICredentialRenewsAndReportsErrors(t *testing.T) {
 	n := 0
-	c := newCLICredential("", "")
+	c := newCLICredential("", "", nil)
 	c.run = func(context.Context, ...string) ([]byte, error) {
 		n++
 		if n == 2 {
@@ -119,3 +123,69 @@ func TestCLICredentialRenewsAndReportsErrors(t *testing.T) {
 		t.Errorf("err = %v", err)
 	}
 }
+
+// jwt is an unsigned token with claims, as az would print it.
+func jwt(claims string) string {
+	enc := base64.RawURLEncoding.EncodeToString
+	return enc([]byte(`{"alg":"none"}`)) + "." + enc([]byte(claims)) + ".sig"
+}
+
+func TestCLICredentialFollowsAzAndRefusesOtherUsers(t *testing.T) {
+	// az's profile has one entry per subscription: signing in as bob,
+	// who also sees s-shared, moved it from alice to bob.
+	profile := []account{
+		{user: "alice@contoso.com", tenant: "t1", anchor: "s-shared"},
+		{user: "bob@contoso.com", tenant: "t1", anchor: "s-bob"},
+	}
+	var args []string
+	issuedTo := "alice@contoso.com"
+	c := newCLICredential("alice@contoso.com", "t1", func() ([]account, error) { return profile, nil })
+	c.run = func(_ context.Context, a ...string) ([]byte, error) {
+		args = a
+		exp := time.Now().Add(time.Hour).Unix()
+		return []byte(fmt.Sprintf(`{"accessToken": %q, "expires_on": %d}`, jwt(`{"upn":"`+issuedTo+`"}`), exp)), nil
+	}
+	arm := policy.TokenRequestOptions{Scopes: []string{"https://management.azure.com/.default"}}
+	if _, err := c.GetToken(context.Background(), arm); err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(strings.Join(args, " "), "--subscription s-shared") {
+		t.Errorf("args %q", args)
+	}
+
+	// Now alice has another subscription, and az would hand out bob's
+	// token for the old one: the new pin is used, and bob's token refused.
+	profile[0].anchor = "s-alice"
+	c.cache = map[string]azcore.AccessToken{}
+	issuedTo = "bob@contoso.com"
+	_, err := c.GetToken(context.Background(), arm)
+	if !strings.Contains(strings.Join(args, " "), "--subscription s-alice") {
+		t.Errorf("args %q", args)
+	}
+	if err == nil || !strings.Contains(err.Error(), "token for bob@contoso.com, not alice@contoso.com") {
+		t.Errorf("err = %v", err)
+	}
+
+	// Alice gone from az entirely: sign in again.
+	profile = profile[1:]
+	if _, err := c.GetToken(context.Background(), arm); err == nil || !strings.Contains(err.Error(), "sign in as alice@contoso.com") {
+		t.Errorf("err = %v", err)
+	}
+}
+
+func TestTokenUser(t *testing.T) {
+	users := []string{"bob@contoso.com"}
+	for claims, want := range map[string]bool{
+		`{"upn":"Bob@Contoso.com"}`:                        true,
+		`{"unique_name":"live.com#bob@contoso.com"}`:       true,
+		`{"upn":"alice@contoso.com"}`:                      false,
+		`{"appid":"00000000-0000-0000-0000-000000000000"}`: false,
+	} {
+		if _, got := tokenUser(jwt(claims), users); got != want {
+			t.Errorf("%s: %v", claims, got)
+		}
+	}
+	if _, got := tokenUser("not-a-jwt", users); got {
+		t.Error("not a JWT")
+	}
+}