Add read/write roundtrip tests

[msto]

Source: Expert Software Review (2026-02-17) — Testing for Software Correctness

Description

There are no end-to-end roundtrip tests that write metrics to a file with MetricWriter and then read them back with Metric.read() to verify the data survives the full cycle. While individual read and write tests exist, a roundtrip test would catch serialization/deserialization mismatches.

Acceptance Criteria

• Roundtrip test for simple metric (str + int fields)
• Roundtrip test for metrics with Optional fields (None preserved)
• Roundtrip test for metrics with list[int] fields
• Roundtrip test for metrics with Counter[StrEnum] fields
• Roundtrip test for empty file (header only, zero rows)

Notes

Could live in tests/test_metric_writer.py or a new tests/test_roundtrip.py.

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Issue ID: ISS-003
Created from: agent_notes/issues/issues_2026-02-17_fgmetric_review.md