fix(packaging): ship py.typed with the four broker distributions

[eaitbrahim]

What

Adds an empty py.typed to keel_broker_api, keel_broker_coinbase, keel_broker_fake and keel_broker_robinhood, plus a packaging test that keeps them there.

Why

All four are checked with strict = true, but none shipped a PEP 561 marker — so every wheel through 0.7.0 carried its annotations invisibly.

Inside this repo it never showed, because mypy reads the source tree. Off it, a consumer sees:

error: Skipping analyzing "keel_broker_api": module is installed, but missing library stubs or py.typed marker

and falls back to Any for every symbol crossing the boundary. The wheel still builds, still installs, still imports — the type contract is just gone. Same silent shape as an unpinned sibling, which is why this lands as a test rather than a note.

keel_broker_api is the one that costs most: the port's types are the contract every adapter and every consumer codes against.

The test

test_strictly_typed_packages_ship_a_py_typed_marker derives its subject list from the [tool.mypy] strict overrides instead of naming packages, so tightening a package into strict mode brings it under the rule with no edit here.

Scoped to strict modules deliberately — keel.* and keel_core.* are still ignore_errors, and a marker on unchecked code promises a guarantee nothing verifies. keel_core carries one anyway for historical reasons; the rule is a floor, not an equality, so that stays legal.

Verification

• Confirmed the test fails when a marker is removed, not just that it passes now.
• Checked the built wheel, not only the source tree: unzip -l keel_broker_api-0.7.0-py3-none-any.whl now lists keel_broker_api/py.typed.
• ruff check keel tests packages clean; mypy clean (224 files); pytest 2721 passed, 1 skipped.

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