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>

clients/web/src/components/groups/ServerSettingsModal/ServerSettingsModal.test.tsx

@@ -445,6 +445,52 @@ describe("ServerSettingsModal", () => {
     });
   });
 
+  // #1906 — a typed endpoint override lands on the settings; clearing it
+  // persists `undefined` rather than an empty string, so "cleared" and "never
+  // set" are the same state on disk.
+  it("persists a typed endpoint override, and clears it back to undefined", async () => {
+    const user = userEvent.setup();
+    const onSettingsChange = vi.fn();
+    const { rerender } = renderWithMantine(
+      <ServerSettingsModal
+        opened
+        settings={emptySettings}
+        serverType="streamable-http"
+        isStdio={false}
+        onClose={vi.fn()}
+        onSettingsChange={onSettingsChange}
+      />,
+    );
+    await user.click(screen.getByRole("button", { name: "OAuth Settings" }));
+    await user.type(
+      screen.getByRole("textbox", { name: /Token URL override/i }),
+      "h",
+    );
+    expect(onSettingsChange).toHaveBeenCalledWith(
+      expect.objectContaining({ oauthTokenUrl: "h" }),
+    );
+
+    onSettingsChange.mockClear();
+    rerender(
+      <ServerSettingsModal
+        opened
+        settings={{
+          ...emptySettings,
+          oauthTokenUrl: "https://staging.test/token",
+        }}
+        serverType="streamable-http"
+        isStdio={false}
+        onClose={vi.fn()}
+        onSettingsChange={onSettingsChange}
+      />,
+    );
+    const clearButtons = screen.getAllByRole("button", { name: /clear/i });
+    await user.click(clearButtons[clearButtons.length - 1]);
+    expect(onSettingsChange).toHaveBeenCalledWith(
+      expect.objectContaining({ oauthTokenUrl: undefined }),
+    );
+  });
+
   // #2018 — the authorization-parameter rows ride the same onOAuthChange
   // callback, so the modal folds them onto the settings object.
   it("persists an added authorization-parameter row onto the settings", async () => {