fix(webview): set cookies for inline HTML on native

[cursor]

Description

Fixes a native WebView regression from PR #2291 where cookieHeader and cookies were silently ignored when rendering inline HTML.

Bug and impact

Trigger: A screen uses EnsembleWebView with the html property plus cookieHeader or cookies on iOS/Android.

Impact: Session/auth cookies are never applied. Inline HTML that depends on authenticated cookies fails to load protected content, causing login loops or broken embedded flows.

Root cause

PR #2291 added html/htmlBaseUrl support via InAppWebViewInitialData, but setCookie() in native/webviewstate.dart still required url to be non-null. Setting html clears url, so cookie setup returned early without configuring the cookie manager.

Fix

• Add resolvedWebViewCookieBaseUrl() that falls back to htmlBaseUrl (or the same default used by native initialData) when url is null.
• Use the resolved base URL in setCookie().
• Add unit tests for the helper.

How to Test

1. From modules/ensemble, run flutter test test/webview_cookie_base_url_test.dart.
2. On a native device/simulator, render a WebView with html, htmlBaseUrl, and cookieHeader; verify the cookie is present for requests from the loaded document.

Duplicate check

• Open PR fix(webview): render inline HTML on web instead of permanent Loading state #2293 (cursor/critical-bug-remediation-a983): fixes web iframe inline HTML rendering — different platform and code path.
• Open PR security(webview): restrict native JS channel callbacks to configured origin #2285 (security): native JS channel origin validation — unrelated.
• Open PR fix(wifi, action): correct callback routing and async apiMap restore #2286: WiFi callback routing + executeAction await — unrelated.
• Open PR fix(cdn): defer lastUpdatedAt commit until manifest fetch succeeds #2284 / fix(cdn): reject manifests missing artifacts before cache wipe #2288: CDN refresh failures — unrelated.
• Merged PR feat(webview): add HTML content and JavaScript injection support #2291: introduced inline HTML on native but did not wire cookie setup for HTML mode — this completes that path.
• No other open branch or PR targets native cookie setup for inline HTML WebViews.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Checklist

• I have run flutter analyze and addressed any new warnings
• I have run flutter test and all tests pass
• I have tested my changes on the relevant platform(s)
• I have updated documentation if needed
• My changes do not introduce new warnings or errors