CLI: fix error handling for invalid access tokens (#5427)

Fixes Cody-3261.
Fixes CODY-2967.
Fixes CODY-2909.

Previously, Cody CLI users got a poor experience if they had provided
expired credentials with `SRC_ACCESS_TOKEN`. The `cody chat` command
reported that the user was "not logged in" and `cody auth login`
confusingly reported that the user was already logged in.

Now, Cody CLI reports a helpful error message saying the user has
invalid credentials and documents how to fix the problem by creating a
new access token.

This fix ended up being harder to implement than I original expected
because the code related to authentication was badly organized.
Specifically, we didn't distinguish between error cases (invalid token)
and unauthenticate cases (the user hasn't logged in yet). I used this
issue as an opportunity to clean up the code so that all the validation
is done in a single function that handles all cases.


## Test plan

Manually tested the following flows

- `cody chat` works with environment variable login and secret storage
login
- `export SRC_ACCESS_TOKEN=foobar`, and confirm that `cody chat` and
`cody auth login` give helpful error messages. Also confirm this happens
when the secret storage has an invalid token (manually edited keychain
item)
```
❯ export SRC_ACCESS_TOKEN=foobar
❯ pnpm agent auth login
...
✖ The provided access token is invalid. The most common cause for this is that the access token has expired. If you are using SRC_ACCESS_TOKEN, create a new token at https://sourcegraph.sourcegraph.com/user/settings/tokens/new?description=CodyCLI and update the value of SRC_ACCESS_TOKEN. If you are using `cody auth login --web`, run `cody auth logout` and try logging in again.
```
- Confirm that `cody auth logout` reports a helpful error message when
`SRC_ACCESS_TOKEN` is set
```
❯ pnpm agent auth logout
...
✖ You cannot logout when using SRC_ACCESS_TOKEN with logout. To fix this problem, run `unset SRC_ACCESS_TOKEN` and try again.
```
<!-- Required. See
https://docs-legacy.sourcegraph.com/dev/background-information/testing_principles.
-->

## Changelog

* Cody CLI now reports a helpful error message when authenticating with
an invalid access token

<!-- OPTIONAL; info at
https://www.notion.so/sourcegraph/Writing-a-changelog-entry-dd997f411d524caabf0d8d38a24a878c
-->

agent/CHANGELOG.md

@@ -10,6 +10,11 @@ This is a log of all notable changes to the Cody command-line tool. [Unreleased]
 
 ### Changed
 
+## 5.5.13
+### Fixed
+
+- `cody chat` and `cody auth login` now report a helpful error message when trying to authenticate with an invalid access token.
+
 ## 5.5.12
 
 ### Fixed

agent/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@sourcegraph/cody",
-  "version": "5.5.12",
+  "version": "5.5.13",
   "description": "Cody CLI is the same technology that powers Cody in the IDE but available from the command-line.",
   "license": "Apache-2.0",
   "repository": {
@@ -17,7 +17,7 @@
     "build:webviews": "pnpm -C ../vscode run -s _build:webviews --mode production --outDir ../../agent/dist/webviews",
     "build": "pnpm run -s build:root && pnpm run -s build:webviews && pnpm run -s build:agent",
     "build-minify": "pnpm run build --minify",
-    "agent": "pnpm run build && node --enable-source-maps dist/index.js",
+    "agent": "pnpm run build:for-tests && node --enable-source-maps dist/index.js",
     "agent:skip-root-build": "pnpm run build:agent && node --enable-source-maps dist/index.js",
     "agent:debug": "pnpm run build && CODY_AGENT_TRACE_PATH=/tmp/agent.json CODY_AGENT_DEBUG_REMOTE=true node --enable-source-maps ./dist/index.js api jsonrpc-stdio",
     "build-ts": "tsc --build",