test(gateway): exercise bundled platform contracts

[kaitwalla]

Summary

• correct the repository root used by bundled-platform discovery
• fail explicitly if the dynamic contract suite discovers no platforms
• let the minimal context accept secondary CLI and lifecycle-hook registrations

Previously the path resolved to tests/plugins/platforms, so both parametrized contract tests skipped silently.

Test plan

• pytest tests/gateway/test_plugin_platform_interface.py -q — 43 passed
• ruff check tests/gateway/test_plugin_platform_interface.py
• git diff --check

This is intentionally separate from the standalone Fluxer plugin extraction requested on #73528.

[teknium1]

Thanks for isolating the contract-test correction. The premise is confirmed on current main: tests/gateway/test_plugin_platform_interface.py:17-18 resolves discovery under tests/plugins/platforms, while bundled adapters live under plugins/platforms/. The added guard prevents this dynamic suite from silently becoming all-skipped, and the mock additions cover existing Photon CLI registration (plugins/platforms/photon/adapter.py:2881) and Raft lifecycle-hook registration (plugins/platforms/raft/adapter.py:846-852).

Automated hermes-sweeper review.