Expand acceptance test coverage (SSH path, -force, skip_create_image, user_data, data source assertions)

Problem

All builder acceptance templates use communicator = "none", so the plugin's core value path is never exercised even when acceptance tests run: temp SSH key generation, Oxide SSH-key create/delete, external IP listing, StepConnect, and provisioning.

Untested configuration surface: -force (both conflict and no-conflict — see oxidecomputer/packer-plugin-oxide#101/oxidecomputer/packer-plugin-oxide#102), skip_create_image, user_data, ip_pool, ssh_public_keys. The data source acceptance test only asserts exit code 0 and never verifies the image_id output value.

Test smells to fix while in here:

• Check functions only assert when buildCommand.ProcessState != nil, so a hung/unstarted command passes silently.
• assertFileContains passes the expected string to regexp.MatchString, so expectations containing regex metacharacters will silently mis-match.

Fix

• Add an acceptance case with communicator = "ssh" and a trivial shell provisioner.
• Add cases for -force, skip_create_image, and user_data.
• Assert data source output values (e.g., via a locals/manifest trick or log assertion).
• Fix the Check/assertFileContains smells.

Start with the builder acceptance templates and the data source acceptance test mentioned in the issue, then inspect the Check functions and assertFileContains helper. Run the acceptance tests while covering SSH, -force, skip_create_image, and user_data cases. Done means the core SSH and provisioning path is exercised, data source image_id output is verified, and hung commands or literal expected strings cannot pass silently.