Add educational tip UI during chat response generation

[Copilot]

Displays rotating educational tips (1-2 sentences) as progress messages while Copilot generates chat responses.

Changes

New service: IChatTipService

• Rotates through 10 predefined tips covering features like @workspace, /explain, #file, inline chat, /tests, etc.
• Registered as singleton in extension services

Stream integration: ChatTipStreamParticipant

• Implemented as ResponseStreamParticipant injected first in DefaultIntentRequestHandler
• Displays tip immediately via stream.progress() with lightbulb icon when chat starts

Implementation

// Service maintains rotating index through tips
class ChatTipService {
  getNextTip(): string | undefined {
    const tip = this._tips[this._currentTipIndex];
    this._currentTipIndex = (this._currentTipIndex + 1) % this._tips.length;
    return tip;
  }
}

// Participant injects tip at stream start
createParticipant(): ResponseStreamParticipant {
  return (inStream: ChatResponseStream) => {
    const tip = this._tipService.getNextTip();
    if (tip) {
      inStream.progress(`$(lightbulb) **Tip:** ${tip}`);
    }
    return inStream;
  };
}

Testing

• 11 unit tests covering rotation, stream behavior, and tip properties

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