Hosting: Invocations channel and Foundry hosted agent sample

[eavanvalkenburg]

Parent: #6265
Depends on: core host and Responses channel; minimal Foundry Hosted Agent helper for the sample portion.

Scope

• python/packages/hosting-invocations/**
• python/samples/04-hosting/af-hosting/foundry_hosted_agent/**

Done when

• Invocations channel works against the simplified host API.
• Tests use tests/hosting_invocations/ with no tests/__init__.py.
• The Foundry hosted agent sample ships with this PR because it uses Responses plus Invocations.
• Package and sample validation pass.