[patch] Fix drush_version field in env:info command

[pwtyler]

Summary

Fixes drush_version field returning blank values in env:info command. drush_version is defined in env:info, but appears to have never worked since the method was introduced.

• Update getDrushVersion() to use appserver_runtime.drush_version instead of deprecated drush_version setting
• Add drush_version to Environment serialize() method for env:info output
• Add test case

🤖 Generated with Claude Code

./bin/terminus env:info --fields=drush_version $MY_SITE.dev
 --------------- ---
  Drush Version   8
 --------------- ---

---

This command only returns the platform configured drush version in pantheon.yml. Most modern Drupal applications (Composer-driven) on Pantheon use the site local Drush instead. It is worth considering whether we should deprecate and remove this field as well.

% ./bin/terminus env:info --fields=drush_version $MY_D11_SITE.dev
 --------------- ----
  Drush Version   10
 --------------- ----
% ./bin/terminus drush $MY_D11_SITE.dev -- --version
Drush Commandline Tool 13.6.2.0

[namespacebrian]

Have we gone through all fields returned by the API when env:info is run to determine if there are any more we might add in the future?

I'd like if we did that now rather than repeatedly discovering new things terminus is already receiving that we'd like to surface to the user.

[namespacebrian]

I feel strongly enough about auditing the fields terminus receives when running env:info to request changes.

[namespacebrian]

Phil points out that this is correcting a field rather than adding a field. Since it hasn't worked for many years, it still seems like adding to me... but I guess I'll drop it for now.