build: suppress per-shard pytest coverage table in unit-tests CI

Each unit-test shard was printing a ~3,700-line per-file coverage
table at the end of its log (the default `--cov=.` behavior is
`--cov-report=term`). The numbers are also misleading per shard
because each shard only runs a subset of the suite.

The dedicated `coverage` job already does the meaningful work: it
downloads every shard's `.coverage` artifact, runs `coverage
combine` + `coverage report` + `coverage xml`, and uploads the
result to codecov. Passing `--cov-report=""` to pytest disables
the terminal report while leaving the `.coverage` data file
intact, so codecov reporting is unaffected and the failure
summary is much easier to find in the CI log.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>