[minor] add php runtime generation to env:list

[pwtyler]

Summary

• Add php_runtime_generation field label to env:list command to expose this field in the output. Field is optional.
• Add test coverage for php_runtime_generation field in env:list functional tests
• Fix indentation in Environment.php getPHPRuntimeGeneration method

Background

This addresses issue #2720 by adding the php_runtime_generation field to the env:list command output. The underlying data was already available in the Environment model's serialize() method, but the field label was missing from the ListCommand, preventing it from being displayed in the output.

Changes

1. src/Commands/Env/ListCommand.php: Added php_runtime_generation: PHP Runtime Generation to the @field-labels annotation
2. tests/Functional/EnvCommandsTest.php: Added test assertion to verify the php_runtime_generation field is present in env:list output
3. src/Models/Environment.php: Fixed indentation in getPHPRuntimeGeneration method

Test plan

• Verify field label is properly added to ListCommand
• Add test coverage for the new field in functional tests
• Run existing test suite to ensure no regressions
• Test env:list command output includes php_runtime_generation field

% ./bin/terminus env:list $MY_SITE --fields=id,php_runtime_generation
 ----------- ------------------------
  ID          PHP Runtime Generation
 ----------- ------------------------
  test        1
  newenv      2
  autopilot   2
  live        1
  dev         2
 ----------- ------------------------
./bin/terminus env:list $MY_SITE --fields="ID","PHP Runtime Generation"
 ----------- ------------------------
  ID          PHP Runtime Generation
 ----------- ------------------------
  test        1
  newenv      2
  autopilot   2
  live        1
  dev         2
 ----------- ------------------------

Resolves #2720