fix(relay): deflake TestSMTPInboundMetric_RejectedLineTooLong

[jiashuoz]

Problem

TestSMTPInboundMetric_RejectedLineTooLong (added in #773) intermittently reds the Go coverage gate and blocks unrelated PRs:

--- FAIL: TestSMTPInboundMetric_RejectedLineTooLong (0.78s)
    metrics_test.go:472: write body: write tcp 127.0.0.1:34262->127.0.0.1:38673: write: connection reset by peer

Seen on main fddf0edf (job). It is flaky, not broken — the same test on recent main: 25356292 pass, bfeec71e pass, 81e8ce87 pass, fddf0edf fail (~1 in 4 in that sample).

Root cause

The test writes a single 160KiB line to exercise the 128KiB MaxLineLength cap. The server hits the cap partway through and resets the connection. Whether the client finishes its Write before the RST arrives depends on socket buffering and scheduling — make cover runs packages at -p 4 on a loaded runner, which is exactly when it loses. The old comment even conceded the assumption ('small enough that the unread remainder … fits in loopback socket buffers'). When the write lost the race, t.Fatalf("write body: …") fired before the real assertion was ever reached.

Fix

Remove the timing dependency instead of re-tuning the body size (a bigger/smaller buffer just moves the threshold and stays racy):

• A w.Write error is no longer fatal — a mid-write reset is itself evidence the server aborted the transaction. It's captured instead.
• The test fails only if both the write and w.Close() succeed — the genuine 'server wrongly accepted a 160KiB line' condition.
• The 554 / 'too long a line' text assertion applies only when the write completed cleanly; after a mid-write reset, Close may surface a bare connection error rather than the SMTP text.
• The deterministic, server-side proof remains assertSingleOutcome(t, metrics, "rejected_line_too_long", false) — the actual thing under test, independent of wire timing, and it runs on every path.

Verification

• go test ./internal/relay -run TestSMTPInboundMetric_RejectedLineTooLong -count=30 — ok (30/30)
• go test ./internal/relay -run TestSMTPInboundMetric -count=10 -p 4 — ok
• go test ./internal/relay -count=1 (whole package) — ok
• Regression check: temporarily raised MaxLineLength to 512KiB in server.go → test fails with DATA with a 160KiB line succeeded; want 554 too-long-line rejection; restored → passes. The flake is fixed without neutering the assertion.
• gofmt -l clean, go vet clean.

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