Remove aiChatSuggestion feature flag

[YoussefKeyrouz]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1212810093780571/task/1213381906205784?focus=true

Description

• Remove the aiChatSuggestions feature flag and treat the AI chat suggestions feature (pinned and recent chats) as always enabled.
• Remove DuckChat.isChatSuggestionsFeatureAvailable() API and inline the always-true branches at every call site (InputScreenFragment, ChatTabFragment, InputScreenViewModel, NativeInputManager, GeneralSettingsViewModel).
• Remove dead helpers (hideAutoCompleteIfOnChatTab, enableViewPagerInputIfNoFavorites) and feature flag related tests.

Steps to test this PR

• Open the input screen, type in chat mode, confirm pinned/recent chats and chat URL suggestions still render
• General Settings shows the chat suggestions toggle when Duck.ai + input screen are enabled. Setting should work as expected when turned on/off.
• Switching between Search and Chat tabs hides/shows overlays as expected
• Enable native input and confirm pinned/recent chat suggestions render under the input

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Note

Medium Risk
Removes a remote-config gate around chat suggestions, so pinned/recent chat suggestions will now activate wherever the input screen/native input supports them; this is user-visible and could affect overlay/focus behavior and suggestion fetching frequency.

Overview
Chat suggestions (pinned/recent chats and related overlays) are now treated as always available by removing the aiChatSuggestions feature flag and the DuckChat.isChatSuggestionsFeatureAvailable() API.

Call sites in the input screen and native input (InputScreenFragment, ChatTabFragment, InputScreenViewModel, NativeInputManager, GeneralSettingsViewModel) were simplified to always run the chat-suggestions code paths, and related dead helper methods/branches were deleted.

Tests were updated accordingly by removing feature-flag expectations and dropping coverage for the “feature not available” scenario.

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[YoussefKeyrouz]

• Remove aiChatSuggestion feature flag #8351 👈 (View in Graphite)
• develop

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[joshliebe]

LGTM 🙂