test(render-foundation): data-URI SVG 解码验证

[leizongmin]

Note

High Risk
Removing BudgetedRenderSession::new in page-load budget rendering can panic at runtime; the dispatch-by-magic test may no longer execute.

Overview
Adds a render-foundation unit test that decodes a URL-encoded data:image/svg+xml… URI via decode_data_uri and checks the SVG rasterizes to a non-zero bitmap (bing-t5 / CSS background-image data-URI path).

The existing magic-byte dispatch test is renamed to test_decode_image_bytes_dispatches_by_magic and is no longer marked #[test], so it may stop running; the new SVG data-URI test has a duplicate #[test] attribute.

In webview advance_render, the line that created BudgetedRenderSession::new(html, css_for_render) after substitute_css_vars_text was removed, while the code still expects a session—likely an accidental regression unless session creation moved elsewhere.

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(requestId: a4ab85d1-89a5-407c-b898-95156f6e7038)