feat(auth): per-server authorization and token URL overrides

Adds two optional per-server fields — `oauth.authorizationUrl` and
`oauth.tokenUrl` — that override the endpoints authorization-server
metadata discovery resolved, so a server can be pointed at a development
or staging authorization server without changing what it advertises.

SDK v2 routes both endpoints through the discovered metadata document and
neither through OAuthClientProvider, so the override is applied by
wrapping the client's base fetch and patching the metadata document in
flight (core/auth/endpointOverrides.ts). Wrapping the base fetch — rather
than only the auth fetch — also covers the discovery the SDK runs from
inside the transport on the 401/refresh path.

The fields round-trip through mcp.json, the /api/servers routes, the
runner (CLI/TUI), and the Server Settings → Authorization panel, where a
value that is not an absolute http(s) URL is flagged inline; at runtime
such a value is dropped with a single warning rather than failing the
connection.

Signed-off-by: cliffhall <cliff@futurescale.com>

AGENTS.md

@@ -45,7 +45,13 @@ v2/main/
 │   │                                   #   providers.redirectToAuthorization, the only
 │   │                                   #   seam that sees the SDK-built authorize URL;
 │   │                                   #   authorization request only, never the token
-│   │                                   #   request — #2018)
+│   │                                   #   request — #2018;
+│   │                                   #   endpointOverrides.ts per-server
+│   │                                   #   authorization/token URL overrides — a fetch
+│   │                                   #   wrapper that rewrites the discovered AS
+│   │                                   #   metadata document, the one seam SDK v2 routes
+│   │                                   #   BOTH endpoints through (neither reaches the
+│   │                                   #   OAuthClientProvider) — #1906)
 │   │   ├── browser/                    # Browser-side OAuth (sessionStorage, BrowserNavigation)
 │   │   ├── node/                       # Node-side OAuth (NodeOAuthStorage, OAuthCallbackServer,
 │   │   │                               #   runner-interactive-oauth loopback callback flow)