Fix for ecctl deployment create bug when --version isn't provided

[mlliarm]

Description

When --version is not provided to ecctl deployment create, the command now automatically fetches the latest available stack version from the API and uses it. This avoids a confusing error when users omit the --version flag.

Key changes in create.go:

• Added a call to getLatestStackVersion() that queries stackapi.List for available stack versions and picks the first (latest) one.
• The auto-fetch is skipped when --file is provided, since file-based payloads carry their own version.
• Prints a message to stderr (--version not set, using latest: X.Y.Z) so the user knows which version was selected.

Related Issues

#756

Motivation and Context

• Previously, omitting --version could lead to an empty version being passed through the deployment creation flow, resulting in unclear errors or unexpected behavior.
• This change makes the CLI more user-friendly by defaulting to the latest available stack version, matching user expectations.
• The --file path is excluded from this behavior since the payload file already specifies its own version.

How Has This Been Tested

• Added a new test case "succeeds creating a deployment without --version by fetching latest stack version" that mocks the stack versions API returning multiple versions (7.8.0, 7.7.0, 7.6.0), verifies the latest is selected, and asserts the stderr message.
• All 30 existing and new tests in cmd/deployment/ pass.
• Test run:

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improves code quality but has no user-facing effect)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Readiness Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[gigerdo]

LGTM 👍 Thanks, great addition for creating deployments.