webview: respect default localResourceRoots for custom editors

[maruthang]

Fixes #282495

Problem

Per the WebviewOptions.localResourceRoots docs, if the option is left unset the webview should default to allowing the workspace folder roots plus the extension's install directory. vscode.window.createWebviewPanel honors this contract — ExtHostWebviewPanels.createWebviewPanel fills in the default before the create RPC.

Custom editors do not. The main thread builds the overlay with contentOptions: {} and then calls $resolveCustomEditor on the extension host. Because the extension host never receives a chance to fill in the default, asWebviewUri returns HTTP 401 for resources under the workspace or the extension folder unless the extension explicitly sets webview.options = { localResourceRoots: [...] }. This silently breaks custom-editor extensions that follow the documented default.

Fix

Added ExtHostWebviews.ensureDefaultContentOptions(handle, contentOptions, extension). When contentOptions.localResourceRoots is undefined, it calls $setOptions with the documented default (workspace folders + extension install directory, mirroring the logic in ExtHostWebviewPanels.createWebviewPanel). When the extension provided explicit roots — including an empty array — the helper does nothing.

MainThreadCustomEditors.$resolveCustomEditor invokes the helper from ExtHostCustomEditors immediately after createNewWebview, before the provider's resolveCustomTextEditor / resolveCustomEditor runs. Existing webview.options = ... assignments inside provider callbacks continue to work — those go through serializeWebviewOptions, which already re-applies the default.

Tests

Added three unit tests in src/vs/workbench/api/test/browser/extHostWebview.test.ts:

• default fills in workspace folders + extension install directory when localResourceRoots is omitted
• explicit localResourceRoots (even []) is respected — $setOptions is not called
• with no workspace open, the default is just the extension install directory

[mjbvz]

Thanks!