SDD: deduplicate startup extension warnings

[moisgobg]

Describe the bug

When starting the CLI, certain extension-related warnings (such as missing configuration settings or the notification that conductor has been replaced by sdd) are displayed twice in the terminal UI.

To Reproduce

1. Have an extension installed that has missing required settings.
2. OR have both the legacy conductor extension installed and the experimental.sdd flag enabled.
3. Start the CLI.
4. Observe the warning bubbles in the UI.

Expected behavior

Each distinct warning should only be displayed once.

Root Cause

The CLI startup flow in gemini.tsx calls loadCliConfig twice. Each call instantiates a new ExtensionManager, which triggers loadExtensions(). This results in coreEvents.emitFeedback('warning', ...) being called twice for the same conditions.

Note on Solution

This is a tactical quick fix that introduces a deduplication Set inside the ExtensionManager to suppress the duplicate UI output. A long-term architectural refactor to avoid double-loading extensions entirely is tracked in #23171.

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Part of #22685

[gemini-cli]

Found possible duplicate issues:

• Duplicate extension warnings in terminal UI during startup #23172

If you believe this is not a duplicate, please remove the status/possible-duplicate label.