Cherry-pick PR #15046: Simplify error message for missing environment variables

[Copilot]

Cherry-picks changes from PR #15046 (merged to release/6.5) to dev branch per sync requirements.

Changes

• Error message update: Simplified MissingEnvironmentVariablesError text from "For new Microsoft 365 Agents Toolkit projects, make sure you've run provision or debug" to "For new projects, run local debugging or provision a remote environment"
• Test cleanup: Removed unused imports (ActionInjector, LocalMcpPrefix) and updated corresponding test assertion in FxCore.test.ts

- "For new Microsoft 365 Agents Toolkit projects, make sure you've run provision or debug to set these variables correctly."
+ "For new projects, run local debugging or provision a remote environment to set these variables."

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Cherry-pick] PR #15046 needs to be synchronized to other branches</issue_title>
<issue_description>

Cherry-pick Reminder

@tecton Hello!

This is an automatically created reminder. Your PR #15046 (fix: error message) is being merged into the release/6.5 branch.

Please ensure to cherry-pick these changes to the following branches:

• cherry-pick fix: error message #15046 to dev

Steps to Follow:

1. Wait for the current PR to be merged.
2. Execute the following commands locally:

   git fetch origin
   git checkout <target_branch>
   git cherry-pick <commit_hash>
   git push origin <target_branch>

3. Or create a new PR to merge these changes into the above branches.

After completion, please check the items above ✓

Note: This is an automatically created issue. You can close this issue after completing all the operations.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Cherry-pick] PR #15046 needs to be synchronized to other branches #15047

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