[BUG] Block external process commands (that spawn az, azd, etc.) from being invoked in remote mode

[anuchandy]

Describe the bug

External process commands (like azqr) use IExternalProcessService to spawn local processes. In HTTP + OBO mode, this is a security risk: processes run under the server's host identity (not the OBO user's context), and malicious requests could exhaust server resources.

Expected behavior

In HTTP + On-Behalf-Of mode, tools that spawn child processes should be disabled.

Actual behavior

Unbounded child processes can be spawned under the server's host identity

Reproduction Steps

1. Start the server in HTTP + OBO mode
2. Invoke a tool that uses external processes (e.g., extension_azqr)
3. Observe child processes spawning under the server's host identity

Environment

HTTP + OBO mode running anywhere