Pass along stderr from failed external auth helper invocations

[benweint]

This change passes along the stderr from an external auth helper, if the helper exits with a non-zero exit status. It also makes the message reported for external auth tool invocation failures more descriptive.

Today, when an external auth helper exits with a non-zero exit code, restish shows an error like this:

With this entry in apis.json:

  "example": {
    "base": "https://httpbin.org",
    "profiles": {
      "default": {
        "auth": {
          "name": "external-tool",
          "params": {
            "commandline": "echo 'bad news' >&2 && exit 1"
          }
        }
      }
    },

... attempting to use this API yields

❯ restish example /status/200
panic: exit status 1

goroutine 1 [running]:
github.com/rest-sh/restish/cli.MakeRequest(0x14000001900, {0x1400069fc30, 0x2, 0x1016be803?})
	/workspace/cli/request.go:244 +0x1298
github.com/rest-sh/restish/cli.Load({0x1400013ed08, 0x13}, 0x140000dec08)
	/workspace/cli/api.go:209 +0x4bc
github.com/rest-sh/restish/cli.Run()
	/workspace/cli/cli.go:821 +0xef4
main.main()
	/workspace/main.go:40 +0x2f4

Which notably lacks the error message produced by the external auth helper. This is obviously a contrived example, but given how finicky auth can be, those errors are often critical to debugging.

Here's how this looks after the change:

❯ restish  example /status/200
panic: external auth tool failed: exit status 1: bad news

... snip ...

[danielgtaylor]

Thanks for the PR. I re-checked this against v2.0.0, and failed external-tool auth now includes a bounded stderr excerpt in the error message.

I verified the focused external-tool auth tests before closing; the v2 path reports errors in the shape:

external-tool auth: tool exited with error: ... stderr: ...

This v1-era patch is superseded by the v2 implementation, so I’m closing it out. Thanks again for pushing on the debugging experience here.