Expand MetricWriter test coverage

[msto]

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

Description

MetricWriter currently has only 1 test (test_writer in tests/test_metric_writer.py). This covers basic write with the default tab delimiter but leaves several code paths untested.

Acceptance Criteria

• Test for writing with custom delimiter (delimiter=",")
• Test for writeall() method standalone
• Test for writing metrics with field aliases (Field(alias=...))
• Test for writing metrics with Counter[StrEnum] fields (pivot columns in output)
• Test for close() behavior (file handle actually closed)
• Test for context manager exception handling (file closed on error)

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Issue ID: ISS-002
Created from: agent_notes/issues/issues_2026-02-17_fgmetric_review.md