[TypeSpec Authoring] Agent unprompted feature inquiry may bypass authoring plan workflow

## Description

In version evolution scenarios, the agent sometimes (approximately 10%~20% of the time) asks the user if there are new features to be added in the new version, even when the user did not prompt or request this.

While asking about new features is not inherently wrong, the issue is that if the user responds with new features to add, the agent may **not** start a new task following the proper workflow — specifically, it may skip calling the authoring plan MCP tool (`azsdk_typespec_generate_authoring_plan`). This can lead to incomplete or incorrect TypeSpec modifications.

## Current Behavior

1. User requests a version evolution (e.g., add new preview/stable version).
2. Agent sometimes (10%~20%) asks: *"Are there any new features you'd like to add in this version?"* — even though the user didn't mention adding features.
3. If the user responds with new features, the agent may proceed to make changes directly without calling the authoring plan MCP tool, bypassing the standard workflow.

## Expected Behavior

- The agent should not proactively ask about new features unless the user explicitly mentions them.
- If the agent does ask and the user provides new features, the agent **must** follow the standard workflow by calling the `azsdk_typespec_generate_authoring_plan` MCP tool to generate a proper authoring plan before making any TypeSpec changes.

## Parent Issue

Part of #14739

Start by tracing the version-evolution authoring workflow and the azsdk_typespec_generate_authoring_plan MCP tool entry point. Reproduce the case where the agent asks about unprompted features, then verify that feature requests trigger the authoring plan before any TypeSpec changes and that unprompted inquiries are avoided.

Written by the indexing model from the issue text.