Use cpex framework package for plugin imports

[lucarlig]

Summary

Plugins should import framework contracts from the standalone cpex framework package, not from the gateway package.

The gateway imports plugins, so plugin packages must not import gateway framework modules. Keeping those imports creates a circular dependency risk between the gateway runtime and standalone plugin packages.

Scope

• Add cpex>=0.1.0.dev11,<0.2.0 as the runtime dependency for every plugin package.
• Replace all gateway framework imports with cpex.framework, cpex.framework.models, and cpex.framework.settings.
• Update plugin tests and test shims to use the standalone cpex framework import surface.
• Update the Rust framework bridge to resolve framework classes from cpex.framework.
• Remove all remaining gateway imports from plugin code and tests.

Validation

• Repo catalog validation should pass.
• Plugin integration suites should pass against the updated standalone framework imports.

Implementation

• Draft PR: Use cpex framework imports #57