Avoid passing ECS session token in plugin argv

[NguyenCong2k]

Summary

• pass the ECS Exec session response to session-manager-plugin through AWS_SSM_START_SESSION_RESPONSE when the installed plugin supports it
• keep the existing argv fallback for older plugin versions
• add unit coverage for the environment-variable path

Test

python -m pytest tests\unit\customizations\test_sessionmanager.py tests\unit\customizations\ecs\test_executecommand_startsession.py -q

[Copilot]

Pull request overview

This PR updates the ECS execute-command integration with session-manager-plugin to avoid placing the StartSession token payload on the process command line when the installed plugin supports passing it via the AWS_SSM_START_SESSION_RESPONSE environment variable, while retaining the argv-based fallback for older plugin versions.

Changes:

• Detect session-manager-plugin version and, for supported versions, pass the StartSession response via AWS_SSM_START_SESSION_RESPONSE instead of argv.
• Preserve the existing argv-based behavior when the plugin version is too old or doesn’t meet the requirement.
• Add unit test coverage for the new environment-variable path in the ECS execute-command flow.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
awscli/customizations/ecs/executecommand.py	Adds plugin version probing and switches to env-var-based StartSession response passing when supported.
tests/unit/customizations/ecs/test_executecommand_startsession.py	Updates existing tests for version probing and adds a new test validating the env-var path.

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[NguyenCong2k]

Thanks, addressed both comments.

• Removed the redundant initial session-manager-plugin check_call; the --version call now serves as the plugin existence check.
• Moved the failing-path argv assertion outside the assertRaises block so it executes.

Tests:

python -m pytest tests\unit\customizations\test_sessionmanager.py tests\unit\customizations\ecs\test_executecommand_startsession.py -q