Hosting: minimal Foundry Hosted Agent helper

[eavanvalkenburg]

Parent: #6265
PR: #6593

Scope

Add the smallest useful FoundryHostedAgentHistoryProvider surface in python/packages/foundry_hosting/agent_framework_foundry_hosting/.

Non-goals

• No broad Foundry hosting rewrite.
• No changes to existing ResponsesHostServer or InvocationsHostServer behavior.
• No linking, multicast, or cross-channel delivery behavior.

Done when

• Later hosting/channel samples can persist and reload Foundry-hosted conversation history through the provider.
• The implementation stays constrained to the provider surface needed by those samples.
• Package validation passes.