fix(cli): resolve infinite loop and output bugs in log merging

[doringeman]

When timestamp1 == timestamp2, neither Before check is true, so neither inner loop executes. The outer loop condition is still true, creating an infinite loop. This happens frequently in practice since the service and engine logs often emit lines at the same time.

Without the second commit, you can easily reproduce this.

$ git reset --hard HEAD~1
HEAD is now at ab173d60 test(cli): add regression test for log merge equal-timestamp deadlock

$ go test ./cmd/cli/commands/ -run TestMergedLog -v
=== RUN   TestMergedLogEqualTimestamps
    logs_test.go:23: printMergedLog hung — equal timestamp deadlock
--- FAIL: TestMergedLogEqualTimestamps (3.00s)
FAIL
FAIL	github.com/docker/model-runner/cmd/cli/commands	3.499s
FAIL

$ git reset --hard HEAD@{1}
HEAD is now at a1baa3c7 fix(cli): resolve infinite loop and output bugs in log merging

$ go test ./cmd/cli/commands/ -run TestMergedLog -v -timeout 10s
=== RUN   TestMergedLogEqualTimestamps
--- PASS: TestMergedLogEqualTimestamps (0.00s)
=== RUN   TestMergedLogInterleavedTimestamps
--- PASS: TestMergedLogInterleavedTimestamps (0.00s)
PASS
ok  	github.com/docker/model-runner/cmd/cli/commands	0.520s

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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[gemini-code-assist]

Code Review

This pull request effectively resolves a critical infinite loop bug in the log merging logic that occurred when log entries had identical timestamps. The fix is correct and also includes a regression test to prevent this issue in the future. Additionally, the changes address an output bug where the last log line could be dropped, and refactors the code to improve testability by using io.Writer and replacing println with fmt.Fprintln. The introduction of new tests significantly improves the robustness of the log merging functionality. I've noted a minor improvement in the new test file to handle potential errors during test setup.