fix(osr): re-assert last-known visibility after browser bind

[wenkaifan0720]

Problem

Under a startup burst of many browser creates, one browser could end up established but permanently blank on-screen. cef_host gates every frame on slot->visible and never reconciles that flag against reality after establishment — so a setVisible edge that races the browser bind (recorded as intent only, browser==null) or is lost/mis-ordered can latch a slot hidden while the tile is on-screen. The native first-present watchdog (DoInvalidate) can't rescue it, because a WasHidden(true) browser's compositor is idled — no Invalidate/BeginFrame produces a frame. Which tile loses is timing-dependent, so a different tile goes blank each launch.

Fix

CefWebController now tracks the last requested visibility and re-asserts it once the browser binds (in _createSession, after channel re-registration) — replaying an edge that landed before bind or was lost under the burst. The false→true re-assert hits the native DoSetVisible edge and repaints the latched tile.

Gated on an explicit-set flag: a slot whose visibility was never set keeps CEF's default-shown, so a fresh/recovered off-screen slot isn't force-repainted (which would pump it at 60fps until its cull edge lands). A matching state is a no-op.

Pairs with a consumer-side change in work_canvas's agent_ui tile, which drives the session's visibility directly (its 5-hop InheritedNotifier path could drop the edge). The direct-drive ensures the correct value is what gets re-asserted; the re-assert here is the generic backstop for all consumers (before-bind window). Both are needed — one closes the before-bind window, the other the after-bind window.

Test

Adds create re-asserts last-known visibility only when explicitly set — pins that an explicit setVisible(false)/(true) is replayed after create(), and a never-set slot is not (the off-screen guard). Full suite green (+59).

Validation

Validated in a consuming app (work_canvas firebase-dev) across 8 relaunches with 16+ CEF tiles: the previously-blank agent_ui tile renders every launch, zero wedges (baseline was ~1 blank tile per launch).

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