Report a malformed --account file instead of panicking

[arpitjain099]

Every value read out of the --account JSON uses a bare type assertion:

details := account["installed"].(map[string]interface{})
authzEp = details["auth_uri"].(string)
...
authzEp = account["auth_uri"].(string)

The file is read and json.Unmarshaled into map[string]interface{} before any network call, so its contents alone decide whether these run. Three shapes panic:

$ step oauth --account sa.json --bare
{"type":"service_account"}   panic: interface conversion: interface {} is nil, not string          cmd.go:452
{"installed":{}}             panic: interface conversion: interface {} is nil, not string          cmd.go:447
{"installed":"notamap"}      panic: interface conversion: interface {} is string, not map[string]... cmd.go:446

Any partial, truncated, or non-Google account file does this. Without STEPDEBUG the panic handler in root.go presents it as "Something unexpected happened" and asks the user to mail the output to info@smallstep.com, so it reads as an internal fault rather than a bad file.

The else branch has a separate bug:

return errors.Wrapf(err, "error reading %s: unsupported account type", filename)

err is nil by then, and errors.Wrapf(nil, ...) returns nil, so an unrecognised account file reported no error and execution continued with empty endpoints. I confirmed that: {"other":1} printed nothing and hung rather than failing.

Each value is now read with the two-value form and names the key that is missing or of the wrong type, and the unsupported-type branch returns a real error:

{"installed":"notamap"}      error reading ...: "installed" must be an object
{"installed":{}}             error reading ...: missing or invalid "auth_uri"
{"type":"service_account"}   error reading ...: missing or invalid "auth_uri"
{"other":1}                  error reading ...: unsupported account type

I moved the block into readAccountCredentials so it can be tested at all. command/oauth had no test file, and driving oauthCmd directly is not a viable test: it launches a browser and starts the real OAuth flow, which I do not want in your CI. The helper is a pure function of the filename, so the nine new tests cover the five error shapes, a missing file, and both well-formed shapes asserting the values and the do2lo flag. They panic without the change.

A well-formed installed file still parses to the same endpoints and credentials as before; the refactor is meant to be behaviour-preserving apart from the error paths.

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