fix: say why the login failed in the v2 authRequired error (#1598)

## Problem

When an API request fails authentication, the v2 agent hides the message
Claude Code writes for it (#1582) and fails the turn with a bare
`Authentication required`. The reason is lost, for example:

> Failed to authenticate: this login reached its time limit and cannot
be renewed. Run `claude auth login` to sign in again, or use `claude
setup-token` and set CLAUDE_CODE_OAUTH_TOKEN for unattended jobs

A client can then only say that the agent is signed out, not why.

## Change

The turn's `authRequired` error now carries the result's error text in
its message: `Authentication required: <text>`. The code and `data` are
unchanged, so a client that starts its own sign-in from the code behaves
as before.

Text that tells the user to run `/login` is still left out, as that
instruction is for Claude Code's terminal (#863). Only the v2 agent
changes.

## Testing

`npm run check`, `npm run build` and `npm run test:run` pass. The v2
tests now pin both messages: the plain one for the `/login` text, and
the one with the reason for an authentication failure reported on the
assistant message.

Not checked against a real expired login.

src/tests/v2/v2.test.ts

@@ -2433,7 +2433,7 @@ describe("ACP v2 core", () => {
     expect(client.sessionUpdates.at(-1)).toMatchObject({
       state: "idle",
       stopReason: "error",
-      error: { code: -32000 },
+      error: { code: -32000, message: "Authentication required" },
     });
   });
 
@@ -2442,7 +2442,7 @@ describe("ACP v2 core", () => {
       "an authentication failure as authRequired",
       "authentication_failed",
       "Failed to authenticate. API Error: 401",
-      { code: -32000 },
+      { code: -32000, message: "Authentication required: Failed to authenticate. API Error: 401" },
     ],
     [
       "another failure with its error kind",

src/v2/outcome.ts

@@ -91,7 +91,11 @@ export function idleOutcome(
       providerFailureCategory(kind) === "auth_required");
   return failed(
     authFailure
-      ? RequestError.authRequired()
+      ? // Claude Code's `/login` instruction is for its terminal, so that text
+        // is left out. Any other says why the login failed, e.g. "Failed to
+        // authenticate: this login reached its time limit and cannot be
+        // renewed."
+        RequestError.authRequired(undefined, ended.loginRequired ? undefined : ended.error)
       : RequestError.internalError(kind ? { errorKind: kind } : undefined, ended.error),
   );
 }