fix(affected): skip a mix test shard whose partition gets no selected file

[calvin-archastro]

What

A narrowed mix test --partitions N shard is skipped when its partition gets none of the selected test files.

Why

mix test --partitions N sorts the files it is given and deals them out in turn. Partition K of fewer than K files is empty, and Mix then fails:

Paths given to "mix test" did not match any directory/file: test/a_test.exs

--related handed every shard the whole selection, whatever its partition. A change that reached one test file therefore failed every shard but the first.

Seen in CI on a private monorepo with four shards: a one-file selection passed in shard 1 in seconds, failed in shards 2 to 4, and each of those was then retried in full (about 10 minutes each).

How

• The partition number is read from MIX_TEST_PARTITION=K written in front of the command in the shard's one-liner, and the total from --partitions N or --partitions=N.
• With fewer than K selected files, the shard has nothing to run and is reported as skipped. The count is enough; the order Mix sorts in does not matter.
• When the partition number is not written in the command (it comes from the caller's environment), nothing changes.

Tests

• New: a_shard_whose_partition_gets_no_selected_file_is_skipped.
• shard_projects_run_their_slice_of_the_selected_tests now selects two files so that both shards run; it previously asserted the failing behaviour for the second shard.
• Replayed the failing pull request's four shard lanes with this build: shard 1 runs the one file, shards 2 to 4 have no targets.
• cargo test: all pass except dev_services::dynamic_port_bundles_are_distinct_propagated_and_released, which fails the same way on main on my machine.
• cargo clippy --all-targets -- -D warnings and cargo fmt --check are clean.

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