[AI Assistant] Remove "Try the new AI Agent" opt-in popover

[jonwalstedt]

Summary

AI Agent Builder is now the default chat experience, making the "Try the new AI Agent" opt-in tour stale and contradictory. This PR removes the popover and all supporting code from both AI assistant implementations.

What was removed:

• kbn-ai-assistant / observability_ai_assistant — AIAgentTourCallout component, useAgentBuilderOptIn hook, useAIAgentTourDismissed hook, related exports from the plugin's public index, and the "Try AI Agent" footer button in ChatActionsMenu.
• kbn-elastic-assistant — AgentBuilderTourStep component (the EuiTourStep-based popover in the Security Assistant), TryAIAgentContextMenuItem, the AI Agent confirmation modal flow in AssistantSettingsContextMenu, and the AGENT_BUILDER_TOUR storage key.

Cascading cleanup:

• isConversationApp prop removed from ChatActionsMenu (was only passed to useAgentBuilderOptIn).
• isInKnowledgeBaseTab prop removed from SelectModelAndInstallKnowledgeBase and its parent chain — it existed solely to shift the EIS callout's anchor position away from the now-removed tour.
• The EisKnowledgeBaseCallout anchor position is now hardcoded to downCenter instead of conditionally leftCenter.
• Five pre-existing EuiPopover accessibility warnings (aria-label missing) fixed across the touched files.

Before fix

After fix

Checklist

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

Identify risks

• Stale localStorage key (elasticAssistant.agentBuilderTour.v9.3): Users who previously dismissed the tour will have a dead key in localStorage. No reader exists after this PR, so there is no functional impact — the key is simply orphaned. Severity: low. Mitigation: none required.
• EIS callout anchor position change: The EisKnowledgeBaseCallout in the knowledge base setup flow previously anchored to leftCenter when the tour was visible and downCenter otherwise. It now always uses downCenter. Severity: low. No workflow is broken; the callout still appears correctly.
• No regressions expected: The removed code was entirely opt-in UI gating. The default experience (AI Agent Builder) is unchanged.

Release note

Removes the "Try the new AI Agent" opt-in popover from the AI Assistant chat header and settings menu, as AI Agent Builder is now the default experience.

Suggested label: release_note:enhancement

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