Remote HTTP MCP server OAuth fails with false "issuer mismatch" when RFC 9728 resource URL differs from authorization server issuer

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS 26.6.2 (25G83)

### Expected behaviour

When connecting to a remote HTTP MCP server that uses OAuth, kiro-cli should follow RFC 9728 (Protected Resource Metadata) and RFC 8414 (Authorization Server Metadata). The MCP resource identifier and the authorization server's issuer are distinct values and are not required to match. The CLI should discover the authorization server listed in the protected-resource metadata and validate its issuer against that authorization server's own metadata document — not against the MCP resource URL. The connection should succeed and complete the OAuth flow, exactly as the Kiro IDE does with the same configuration.

### Actual behaviour

kiro-cli rejects the connection with:

GitLab ✗ failed Authorization server issuer mismatch:
expected https://X.gitlab-dedicated.com/api/v4/mcp,
received https://X.gitlab-dedicated.com

The CLI incorrectly expects the authorization server's issuer to equal the MCP resource URL (.../api/v4/mcp). GitLab correctly advertises a resource of https://X.gitlab-dedicated.com/api/v4/mcp with an authorization server / issuer of https://X.gitlab-dedicated.com. These are supposed to differ per spec, so the CLI raises a false mismatch and refuses to connect. The identical configuration works in the Kiro IDE.

### Steps to reproduce

1. Configure a remote HTTP MCP server in ~/.kiro/settings/mcp.json that uses OAuth where the authorization server issuer differs from the MCP resource URL. Example (GitLab Dedicated):

"GitLab": {
"type": "http",
"url": "https://X.gitlab-dedicated.com/api/v4/mcp",
"oauthScopes": ["mcp"]
}

2. The server's OAuth discovery returns (both spec-compliant):
- GET /.well-known/oauth-protected-resource/api/v4/mcp
{ "resource": "https://X.gitlab-dedicated.com/api/v4/mcp",
"authorization_servers": ["https://X.gitlab-dedicated.com"],
"scopes_supported": ["mcp"] }
- GET /.well-known/oauth-authorization-server
{ "issuer": "https://X.gitlab-dedicated.com", ... }

3. Start a chat / trigger the MCP connection:
kiro-cli chat

4. Observe the "Authorization server issuer mismatch" error and the server failing to connect.

Note: The same config connects successfully in the Kiro IDE, confirming the discovery documents are valid and the defect is in the CLI's issuer validation.

### Environment

```yaml

[q-details]
version = "2.19.2"
hash = "80d5aeb353825e5fbbe66c5e5c07084979af0262"
date = "2026-08-25T02:48:27.049726Z (2d ago)"
variant = "full"

[system-info]
os = "macOS 26.6.2 (25G83)"
chip = "Apple M4 Pro"
total-cores = 14
memory = "24.00 GB"

[environment]
cwd = "/Users/USER"
cli-path = "/Users/USER"
os = "Mac"
shell-path = "/bin/zsh"
shell-version = "5.9"
terminal = "iTerm 2"
install-method = "unknown"

[env-vars]
PATH = "/Users/USER/.local/bin:/Library/Frameworks/Python.framework/Versions/3.10/bin:/opt/homebrew/bin:/opt/homebrew/sbin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/pkg/env/global/bin:/Applications/iTerm.app/Contents/Resources/utilities:/Users/USER/.local/bin"
QTERM_SESSION_ID = "b40ddb9c90db46f1aafbc571a36d5564"
Q_SET_PARENT_CHECK = "1"
Q_TERM = "2.15.2"
SHELL = "/bin/zsh"
TERM = "xterm-256color"
__CFBundleIdentifier = "com.googlecode.iterm2"
```

Beginne mit der Remote-HTTP-MCP-Konfiguration in ~/.kiro/settings/mcp.json und reproduziere den Fehler mit `kiro-cli chat` anhand des GitLab Dedicated-Beispiels. Verfolge die Discovery der OAuth-geschützten Ressource und des Authorization-Servers und überprüfe anschließend, dass eine sich vom Issuer unterscheidende Ressourcen-URL den OAuth-Flow ohne die falsche Abweichung abschließt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.