Remove the obsolete standalone Duck.ai screen and its rollout flag

[malmstein]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1174433894299346/task/1217189225202021
Tech Design URL (if applicable):
API Proposals URL(s) (if applicable): None

Description

Stacked on #9394 (collapse InputModeWidget), which is stacked on #9392, which is stacked on #9391. Review and merge those first — this PR targets #9394's branch, so the diff here is only the fourth step.

Deletes the overlay-based fullscreen Duck.ai experience (DuckChatWebViewFragment/DuckChatWebViewViewModel, a fragment that slid in over the browser from the right) and the showFullScreenMode rollout flag that gated it. The flag's replacement — tab-based Duck.ai, opened via BrowserActivity.launchDuckAi()'s new-tab path — is already the only reachable behaviour once this lands; the flag defaulted INTERNAL and every non-internal caller already took that branch by the time this task was scoped, so there's no user-visible change.

Four commits:

1. Delete DuckChatWebViewFragment/DuckChatWebViewViewModel and their layout/test. Nothing routes to them any more.
2. Remove the flag and its infrastructure — showFullScreenMode/showFullScreenModeToggle off DuckAiFeatureState, DuckChatFeature.fullscreenMode(), the DUCK_CHAT_FULLSCREEN_MODE_SETTING preference and its accessors. isDuckChatFullScreenModeEnabled() is a distinct, still-live accessor (feeds DuckChatJSHelper's JS capability signalling) — it now tracks Duck.ai feature availability directly rather than the retired rollout computation, since that's the only gate left with any meaning once the fragment alternative is gone.
3. Collapse every showFullScreenMode.value branch in :app to its true side: BrowserActivity.launchDuckAi()/CLOSE_DUCK_CHAT handling, BrowserViewModel.openDuckChat()/Command.OpenDuckChat, BrowserTabViewModel.evaluateDuckAIPage()/openDuckAiQuery()/openDuckAiChatById()/onDuckChatOmnibarButtonClicked(), TabSwitcherViewModel.onDuckAIButtonClicked(), and SpecialUrlDetector's ShouldLaunchDuckChatLink special-casing (only reachable when the flag was off). ViewTransitionAnimations.kt (bundled into commit 1) existed solely for the overlay's slide-in/out, so it goes with the last caller.
4. Regenerate the app lint baseline (./gradlew :app:updateLintBaseline) to drop the stale NoImplImportsInAppModule entries for the deleted import and reconcile line numbers.

Deviations from the original task notes worth flagging in review

• isDuckChatFullScreenModeEnabled()'s new definition (isDuckChatFeatureEnabled instead of the rollout-flag computation) wasn't in the task's listed steps — it fell out of removing fullscreenMode(), since this accessor is a separate, still-consumed concept from the showFullScreenMode StateFlow the task named.
• The task's Step 5 said "two menu click handlers" checked the flag in BrowserTabFragment; only one still did (the other already called openNewDuckChat unconditionally) — the codebase moved since the task was scoped.
• One pre-existing, previously-untested quirk surfaced once the true branch became unconditional: BrowserTabViewModel.openDuckAiQuery() fires onInputSubmitted() on browser-interaction plugins twice (once directly, once via the onUserSubmittedQuery() call it routes through). This predates the flag removal — it was already true whenever showFullScreenMode was on — so it's documented on the test rather than fixed here, to keep this PR a pure flag removal.
• A prior attempt at this exact task (#8134, linked from the Asana task's "First attempt" subtask) hardcoded showNewDuckChatTabOption = false, breaking the live new-Duck.ai-tab feature instead of just the dead overlay — a mix-up between the fragment's own showFullScreenMode-gated branches and the unrelated, still-live hasSessionActive concept threaded through BrowserNav.openDuckChat(). This PR verified each branch's kept side against production before collapsing it, and left BrowserNav/AppBrowserNav/RealDuckChat/the widget shortcut call sites untouched.

Steps to test this PR

• Tap the omnibar Duck.ai button (unfocused, browser tab): opens a new Duck.ai tab
• Type in the omnibar, tap the Duck.ai chevron while focused: opens Duck.ai with the typed query pre-filled
• New tab page, tap "Ask Duck.ai" / submit a query with the Duck.ai toggle selected: opens in a new tab, NTP tab itself is reused (not duplicated)
• Tab switcher FAB → Duck.ai: opens a new Duck.ai tab and closes the switcher
• Navigate to a duck.ai URL directly (address bar or link): loads normally in the tab, no special-cased redirect
• Contextual sheet still opens from the omnibar when unfocused and not on NTP (unrelated showContextualMode path, untouched)
• CLOSE_DUCK_CHAT intent (e.g. widget/notification action) while in a Duck.ai tab: closes the tab

UI changes

None.

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Note

Medium Risk
Touches many Duck.ai navigation paths across browser and duckchat modules; regressions could affect how users open or close Duck.ai tabs, though behavior is intended to match the flag-on path already in production.

Overview
Removes the obsolete overlay fullscreen Duck.ai experience (DuckChatWebViewFragment / view model, layout, tests) and the showFullScreenMode rollout path, including DuckChatFeature.fullscreenMode(), the fullscreen preference, and showFullScreenMode / toggle fields on DuckAiFeatureState.

Duck.ai entry points in :app no longer branch on that flag: BrowserActivity.launchDuckAi() always opens a new Duck.ai tab, CLOSE_DUCK_CHAT closes the current tab when it is Duck.ai, and omnibar / NTP / tab-switcher handlers route through the new-tab navigation helpers in BrowserTabViewModel and BrowserTabViewModel. Overlay-only slide animations in ViewTransitionAnimations are deleted with the last caller.

isDuckChatFullScreenModeEnabled() remains for JS capability signalling in DuckChatJSHelper; it now mirrors Duck.ai feature availability instead of the retired rollout computation. App lint baseline is regenerated for deleted imports and line shifts.

No intended user-visible change: tab-based Duck.ai was already the effective behavior when the flag was on. A pre-existing double onInputSubmitted() call in openDuckAiQuery() is left as-is.

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[YoussefKeyrouz]

Tested. All looks good. Thanks for cleaning this.