Fix pull command to show only final layer completion

[Copilot]

The pull command was displaying downloading progress and completion messages for all layers. Expected behavior is to show only the final layer's completion status.

Before:

$ model-cli pull ai/smollm2:135M-Q4_K_M
ed5fa30c487b: Downloading [=================================================> ]  105.3MB/105.5MB
ed5fa30c487b: Pull complete
609e2cb599f8: Downloading [================>                                  ]  4.096kB/12.62kB
609e2cb599f8: Pull complete
Model pulled successfully

After:

$ model-cli pull ai/smollm2:135M-Q4_K_M
609e2cb599f8: Pull complete
Model pulled successfully

Changes

• Modified DisplayProgress() to track the last completed layer instead of emitting progress for all layers
• Added writeDockerComplete() helper to emit a single completion message after stream processing completes
• Added test coverage for multi-layer progress display behavior

The implementation buffers layer completion tracking and emits only the final layer's status, matching Docker's behavior for image pulls.

Original prompt

On completion of a pull this should be:

$ cmd/cli/model-cli pull ai/smollm2:135M-Q4_K_M
609e2cb599f8: Pull complete
Model pulled successfully

Instead it's currently:

$ cmd/cli/model-cli pull ai/smollm2:135M-Q4_K_M
ed5fa30c487b: Downloading [=================================================> ] 105.3MB/105.5MB
ed5fa30c487b: Pull complete
609e2cb599f8: Downloading [================> ] 4.096kB/12.62kB
609e2cb599f8: Pull complete

Model pulled successfully

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