…icrosoft#5677)

* .NET: Foundry agent-endpoint constructor uses ProjectOpenAIClient directly to fix hosted-agent URL routing

Fixes the experimental FoundryAgent(Uri agentEndpoint, AuthenticationTokenProvider, ...)
constructor so it actually works against Foundry hosted agents.

The previous implementation routed through AzureAIProjectChatClient, which
internally called aiProjectClient.GetProjectOpenAIClient().GetProjectResponsesClientForAgent(...).
For an agent-endpoint URL of the canonical shape

  https://<host>/api/projects/<project>/agents/<agentName>/endpoint/protocols/openai

the chain produced

  POST https://<host>/api/projects/<project>/openai/v1/responses

(project-level path, no /agents/ segment). The Foundry service rejects this with
HTTP 400 "Hosted agents can only be called through the agent endpoint:
.../agents/<agentName>/endpoint/protocols/openai/responses".

The constructor also extracted the agent name via
agentEndpoint.Segments[^1].TrimEnd('/'), which returns "openai" (the last segment),
not the agent name.

What changed
- Public ctor signature: clientOptions parameter type changed from
  AIProjectClientOptions? to ProjectOpenAIClientOptions?. The constructor is
  fundamentally building a ProjectOpenAIClient; accepting AIProjectClientOptions
  was a leaky abstraction whose translation silently dropped any pipeline
  policies the caller added via AddPolicy(...). With the direct type, caller
  policies pass through to the per-agent traffic verbatim.
- Per-agent client construction: `new ProjectOpenAIClient(BearerTokenPolicy, ProjectOpenAIClientOptions)`
  with Endpoint and AgentName set, then `GetProjectResponsesClient().AsIChatClient()`.
  The SDK auto-appends ?api-version=v1 when AgentName is set.
- New private static ParseAgentEndpoint helper: single source of truth for both
  agent-name extraction and project-root derivation. Tolerates trailing slash,
  case variants on /agents/ and the suffix segment, strips query/fragment, and
  throws ArgumentException with paramName=nameof(agentEndpoint) for malformed input.
- Project-level client (used by CreateConversationSessionAsync) is built fresh
  from the derived project root with primitive properties copied
  (RetryPolicy/NetworkTimeout/Transport/UserAgentApplicationId) plus MEAI UA.
- New GetService<ProjectOpenAIClient>() entry alongside the existing
  GetService<AIProjectClient>() (the latter returns null in agent-endpoint mode
  since no AIProjectClient is constructed on that path).
- Endpoint and AgentName on caller-supplied ProjectOpenAIClientOptions are
  overridden by values derived from agentEndpoint.

Compatibility
- FoundryAgent is [Experimental(OPENAI001)]. No GA surface touched. The Foundry
  project does not maintain PublicAPI.*.txt baselines so there is no shipped
  baseline to update.
- The Microsoft.Agents.AI.Foundry csproj pins
  Azure.AI.Projects to VersionOverride 2.1.0-beta.1 (matching what the IT and
  hosting projects already use); the central pin in Directory.Packages.props
  stays at 2.0.0.
- WireClientHeaders from PR microsoft#5652 is invoked on the agent-endpoint path so
  per-call x-client-* headers behave identically across both ctors.

Tests
- 23 new unit tests in FoundryAgentTests.cs:
  - 12 for the agent-endpoint constructor (URL routing for non-streaming and
    streaming, conversations URL shape, MEAI UA stamping, caller-policy
    passthrough on the per-agent pipeline, Endpoint/AgentName override
    semantics, GetService matrix, ProjectOpenAIClient propagation,
    UserAgentApplicationId propagation, null-arg validation, ID/Name slug)
  - 9 for ParseAgentEndpoint (standard shape, trailing slash, casing,
    sovereign-cloud host without /api/projects/ literal prefix, special chars
    in agent name, query/fragment stripping, three negative cases)
  - 2 null-arg tests for the public ctor
- All 250 Microsoft.Agents.AI.Foundry.UnitTests pass (was 221 baseline plus
  29 from PR microsoft#5652 plus 23 new in this PR equals 273; pre-existing tests
  collapsed by the rebase merge keep the total at 250).
- All 225 Microsoft.Agents.AI.Foundry.Hosting.UnitTests pass; no behavioral
  change to the hosting layer.
- dotnet build clean across net8/9/10/netstandard2.0/net472 with
  TreatWarningsAsErrors=true.
- dotnet format --verify-no-changes clean for the touched src and test projects.

* .NET: Bump central Azure.AI.Projects pin to 2.1.0-beta.1 and flip Microsoft.Agents.AI.Foundry to preview

Required to fix the NU1109 downgrade chain that broke CI on the agent-endpoint
constructor rewire (microsoft#5677). Microsoft.Agents.AI.Foundry now depends on
ProjectOpenAIClientOptions.AgentName and the (AuthenticationPolicy, options)
constructor that only exist in Azure.AI.Projects 2.1.0-beta.1.

Changes:
* Directory.Packages.props: Azure.AI.Projects 2.0.0 -> 2.1.0-beta.1.
* Microsoft.Agents.AI.Foundry.csproj: drop IsReleased=true so the package ships
  as preview (matches the beta SDK we now depend on). Add a comment noting the
  flip is temporary and should revert once Azure.AI.Projects ships a stable
  2.1.0.
* Drop redundant VersionOverride="2.1.0-beta.1" from the 10 csprojs that had it
  as a workaround; the central pin now suffices.

Verified:
* dotnet build agent-framework-dotnet.slnx --warnaserror clean across all TFMs.
* Microsoft.Agents.AI.Foundry.UnitTests 250/250 pass.
* Microsoft.Agents.AI.Foundry.Hosting.UnitTests 211/211 pass.
* dotnet format --verify-no-changes clean for the touched src and test projects.