Fix invalid --plain-http flag on oras cp

[bschwedler]

oras cp has two endpoints, so unlike every other oras subcommand it takes --from-plain-http and --to-plain-http rather than a single --plain-http. OrasCopy.command emitted the bare form whenever plain_http was set:

$ oras cp --plain-http localhost:5601/src:v1 localhost:5602/dest:v1
Error: unknown flag: --plain-http

Confirmed against oras 1.3.3, whose cp --help lists only the per-endpoint flags. Sibling subcommands (manifest fetch, manifest index create) do accept --plain-http, so OrasCommand's shared field is correct everywhere except OrasCopy.

Each flag is now emitted only when that end is actually a registry — an OCI layout is a filesystem path, and a plain-HTTP flag aimed at it is meaningless:

from_oci_layout	to_oci_layout	Flags
false	false	--from-plain-http --to-plain-http
true	false	--to-plain-http only
false	true	--from-plain-http only

Impact

Production is unaffected — plain_http is only set for local registries. But any local-registry publish path failed immediately, which covers local development, the oven, and integration tests publishing against a registry:3 container.

Why the tests did not catch it

test_copy_with_plain_http covered this flag and passed, because it only asserted command construction — oras is never invoked, so it asserted the broken form was built correctly.

That gap is not fully closed here. Unit tests cannot validate flag validity; only running oras can. This PR updates the assertion, adds a regression guard that no layout combination emits the bare flag, and covers dropping the flag aimed at a layout endpoint. An integration test that actually invokes oras against test/helpers/registry_container.py would be the real fix, and is worth considering separately.

Verification

• ruff check and ruff format --check clean
• pytest test/plugins/builtin/imagetools/ → 134 passed
• The single failure, TestFindOrasBin::test_find_from_path, is pre-existing and environmental (oras not on PATH); confirmed by running the same suite on unmodified code (60 passed → 62 passed with this change, same one failure)

Found while validating the retained-OCI-layout publish change.

• Publish from the retained SOCI OCI layout instead of re-downloading from the temp registry #772

[github-actions]

Test Results

2 258 tests   2 258 ✅  11m 37s ⏱️
    1 suites      0 💤
    1 files        0 ❌

Results for commit a6c5429.