refactor: split "basic" and "oauth2" authentication impl into smaller single-purpose components (#12236)

Forgejo's `basic` and `oauth2` authentication methods perform five distinct types of authentication:
- Username and password authentication
- Personal access tokens
- OAuth2 access tokens
- Forgejo Action's `${{ forgejo.token }}` -- task-based static tokens
- Forgejo Action's `${{ env.ACTIONS_RUNTIME_TOKEN }}` JWT, which is the authentication method used for `upload-artifact` (mirroring GitHub's implementation)

`basic` and `oauth2` both supported almost all of these methods, resulting in quite a bit of code duplication between them.  This PR splits personal access tokens into `access_token.go`, Action's task-based tokens into `action_task_token.go`, and Action's JWT tokens into `action_runtime_token.go`.

**Note:** There is one peculiar side-effect that is worth discussing.  Previously, `Authorization: Basic ...` was handled by one complex code path in basic.go, and `Authorization: Bearer ...` was handled by another in oauth2.go, and if authorization failed and a 401 was returned, a single error message would be returned to the user.  Now, as multiple authorization methods may look at `Authorization: Basic ...` and provide their own reason why authorization didn't work, a 401 response has multiple reasons for a lack of authorization listed:

```
401 Unauthorized
...

failure to authenticate with oauth2 access token: not a JWT
Basic authorization is not allowed while having security keys enrolled
access token does not exist [sha: notpassword]
task with token "notpassword": resource does not exist
```

A couple tests have been adapted to check that the result contains their expected response, rather than is equal-to or prefixed-with their expected result.  This is caused by the "auth group" joining together any "invalid credentials" errors, and, to a certain extent it is useful to understand why the authorization request failed.  But it's a bit obscure as well.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. All work and communication must conform to Forgejo's [AI Agreement](https://codeberg.org/forgejo/governance/src/branch/main/AIAgreement.md). There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests for Go changes

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
  - Relying on integration testing for regression checks.
- I ran...
  - [x] `make pr-go` before pushing

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12236
Reviewed-by: Andreas Ahlenstorf <aahlenst@noreply.codeberg.org>

tests/integration/api_twofa_test.go

@@ -6,6 +6,7 @@ package integration
 import (
 	"fmt"
 	"net/http"
+	"slices"
 	"strings"
 	"testing"
 	"time"
@@ -82,7 +83,7 @@ func TestAPIWebAuthn(t *testing.T) {
 
 	DecodeJSON(t, resp, &userParsed)
 
-	assert.Equal(t, "Basic authorization is not allowed while having security keys enrolled", userParsed.Message)
+	assert.Contains(t, userParsed.Message, "Basic authorization is not allowed while having security keys enrolled\n")
 }
 
 func TestAPIWithRequiredTwoFactor(t *testing.T) {
@@ -144,7 +145,14 @@ func TestAPIWithRequiredTwoFactor(t *testing.T) {
 			var response userResponse
 			DecodeJSON(t, resp, &response)
 
-			assert.True(t, strings.HasPrefix(response.Message, messagePrefix))
+			assert.True(t,
+				slices.ContainsFunc(
+					strings.Split(response.Message, "\n"),
+					func(msg string) bool {
+						return strings.HasPrefix(msg, messagePrefix)
+					},
+				),
+				"expected prefix %q, but response message was %q", messagePrefix, response.Message)
 		}
 	}