.NET: Add Hosted-MemoryAgent sample with isolation key plumbing (#5692) (#5702)

* .NET: Add Hosted-MemoryAgent sample with isolation key plumbing (#5692)

Adds HostedSessionContext + HostedSessionIsolationKeyProvider in Microsoft.Agents.AI.Foundry.Hosting so AIContextProviders (notably FoundryMemoryProvider) can scope per user via the platform's x-agent-user-isolation-key / x-agent-chat-isolation-key headers.

- New types: HostedSessionContext (sealed), HostedSessionContextExtensions (public Get, internal Set), abstract HostedSessionIsolationKeyProvider (async), internal PlatformHostedSessionIsolationKeyProvider mapping ResponseContext.Isolation.

- AgentFrameworkResponseHandler now resolves the provider, tags fresh sessions, and validates resumed sessions against the live request (strict 403 'Hosted session identity context mismatch' on any mismatch; 500 on null keys).

- New shared sample project Hosted_Shared_Contributor_Setup hosts DevTemporaryTokenCredential and DevTemporaryLocalSessionIsolationKeyProvider plus AddDevTemporaryLocalContributorSetup. All 9 existing responses samples migrated to consume it so local runs keep working under the strict isolation contract.

- New Hosted-MemoryAgent sample: travel assistant wired through FoundryMemoryProvider with stateInitializer reading session.GetHostedContext().UserId. Includes Dockerfile, smoke.ps1, agent.yaml/manifest.

- New IT scenario 'memory' in Foundry.Hosting.IntegrationTests + MemoryHostedAgentFixture + MemoryHostedAgentTests. Verified end to end against the tao Foundry project.

- ADR 0026 captures the design tree.

* Address PR review feedback

- Dockerfile: add header noting it targets NuGet builds; contributors must use Dockerfile.contributor for ProjectReference source builds.

- PlatformHostedSessionIsolationKeyProvider: doc said 'returns context with empty values'; corrected to 'returns null' which the handler treats as 500.

- FakeHostedSessionIsolationKeyProvider: doc clarifies that null configurations are allowed for testing the handler error path.

- HostedSessionContextExtensions.SetHostedContext: enforce write-once with InvalidOperationException; doc + xml exception updated.

- AgentFrameworkResponseHandler: cache PlatformHostedSessionIsolationKeyProvider as static readonly to avoid per-request allocation.

- MemoryHostedAgentTests: tighten waits from 20s to 5s (FoundryMemoryProvider defaults UpdateDelay=0; ingestion ~3s).

- Sample Program.cs imports reordered to satisfy IDE0005.

* Add HostedFoundryMemoryProviderScopes built-in helpers (#5692)

Addresses review feedback from @lokitoth on Hosted-MemoryAgent/Program.cs:54.

- New HostedFoundryMemoryProviderScopes static class with PerUser, PerChat, PerUserAndChat factories returning Func<AgentSession?, FoundryMemoryProvider.State>.

- All helpers throw InvalidOperationException when GetHostedContext() is null, with a message pointing at writing a custom stateInitializer for non-hosted scenarios.

- New HostedFoundryMemoryScope enum and AddHostedFoundryMemoryProvider DI extension (two overloads: explicit AIProjectClient and DI-resolved). Singleton lifetime. Default scope = PerUser.

- Hosted-MemoryAgent sample and the memory IT scenario container both swap their inline lambdas for HostedFoundryMemoryProviderScopes.PerUser().

- 14 new unit tests (241/241 hosting unit tests pass).

* Replace HostedFoundryMemoryScope enum with Func<...> parameter (#5692)

Address PR review feedback from @westey-m: enums are a breaking-change hazard when extended, and the enum was redundant with the existing HostedFoundryMemoryProviderScopes static class.

- Delete HostedFoundryMemoryScope.cs.

- AddHostedFoundryMemoryProvider DI extensions now take Func<AgentSession?, FoundryMemoryProvider.State>? stateInitializer = null. When null, default to HostedFoundryMemoryProviderScopes.PerUser().

- Callers pick a built-in helper (PerUser/PerChat/PerUserAndChat) or pass a custom delegate. New built-ins are a single static method addition with zero impact on existing callers.

- Tests updated; 244/244 hosting unit tests pass.

* Fix isolation context resume for externally-created conversations (#5692)

Branch on the session's existing hosted-context (not on conversation_id presence) so a conversation provisioned externally (e.g. via conversations.CreateProjectConversationAsync) is treated as fresh on first hosted-agent request and stamped, rather than rejected with 403 hosted_session_identity_mismatch. Strict equality is preserved on real resume of an already-stamped session.

Also tighten dotnet/global.json to version 10.0.204 + rollForward latestPatch so local builds match the CI Docker image SDK and avoid 10.0.300 dotnet format stripping required usings.

* Revert global.json SDK pin to upstream (#5692)

The 10.0.204 + latestPatch pin from the previous commit broke the dotnet-format CI job (hostfxr_resolve_sdk2 could not find a compatible SDK in the mcr.microsoft.com/dotnet/sdk:10.0 image). Restore upstream 10.0.200 + minor; local Release builds with SDK 10.0.300 should set GITHUB_ACTIONS=true to bypass the auto-format-on-build target.