[Architecture] Replace OPENAICUA001 experimental diagnostic with the standard OPENAI001 - #1245

OpenAI .NET 2.13.0 gates its computer use types with OPENAI001
(openai/openai-dotnet#1245), so the pragmas in hosting code and its
converter tests were no-ops. OPENAICUA001 now only marks MAF's own
computer tool factories.

OpenAI .NET 2.13.0 retired OPENAICUA001 in favor of OPENAI001
(openai/openai-dotnet#1245). Both FoundryAITool.CreateComputerTool
overloads now use the shared OPENAI001 id, so callers need no extra
suppression; remaining OPENAICUA001 pragmas and NoWarn entries go.

…crosoft#8756)

* feat(dotnet): add GA computer tool factory

- Parameterless FoundryAITool.CreateComputerTool() emits the GA
  {"type":"computer"} tool; the (env, w, h) overload stays preview
- OpenAI .NET 2.13/2.14 has no GA factory, so the tool is read from
  its wire JSON via ModelReaderWriter as a stopgap
- Both overloads use OPENAICUA001 through a shared DiagnosticIds const

* feat(dotnet): map computer items in Foundry hosting

Hosting dropped computer_call and computer_call_output from input and
history, and never emitted the model's computer_call to the caller.

- Convert items between AgentServer and OpenAI models through JSON,
  stripping AgentServer envelope fields and using builder item ids
- Rebuild AgentServer calls without pending_safety_checks, which
  otherwise throw NullReferenceException on serialization (beta.8)

Known blocker, not worked around: OpenAI .NET replays a GA call with
"action": null and "pending_safety_checks": [], both rejected by the
Responses API, so hosted turns after a computer call fail.

* docs(dotnet): add GA computer use sample

Foundry project endpoint rejects {"type":"computer"} today, so the
sample targets public OpenAI with the Foundry setup commented out.

* chore(dotnet): drop stale OPENAICUA001 pragmas

OpenAI .NET 2.13.0 gates its computer use types with OPENAI001
(openai/openai-dotnet#1245), so the pragmas in hosting code and its
converter tests were no-ops. OPENAICUA001 now only marks MAF's own
computer tool factories.

* refactor(dotnet): gate computer tool factories with OPENAI001

OpenAI .NET 2.13.0 retired OPENAICUA001 in favor of OPENAI001
(openai/openai-dotnet#1245). Both FoundryAITool.CreateComputerTool
overloads now use the shared OPENAI001 id, so callers need no extra
suppression; remaining OPENAICUA001 pragmas and NoWarn entries go.

* docs(dotnet): rename computer use samples

GA computer tool is the default going forward, so it takes the
unsuffixed name (Agent_Step27_ComputerUse) and the existing preview
sample becomes Agent_Step15_ComputerUsePreview.

* docs(dotnet): renumber GA computer use sample to Step15.2

Keeps the GA sample next to its preview counterpart, following the
existing Step02.1/Step02.2 numbering.

* docs(dotnet): narrow computer call replay caveat to action

The service rejects a resent GA computer_call for its "action": null,
which is what the known limitation now describes; the known-issue test
asserts only that key.

* fix(dotnet): resend GA computer calls without action

OpenAI .NET models only the preview computer_call and writes
"action": null next to "actions" when a GA call is sent back, which
the Responses API rejects. Foundry hosting resends history on every
turn, so the turn after a computer call failed.

Replayed calls now use a ComputerCallResponseItem subclass that writes
the call JSON as received; typed properties stay available.

* docs(dotnet): move GA computer use sample to Azure providers

The Foundry project Responses endpoint does not accept the GA
computer tool yet, so the GA sample now targets the Azure OpenAI
endpoint with Entra ID as Agent_With_AzureOpenAIResponses_ComputerUse.
The Foundry sample keeps the preview tool as
Agent_Step15_ComputerUsePreview.

* style: Simplify JsonElement qualification

* fix(dotnet): run GA computer sample through Foundry

* fix(dotnet): isolate computer sample assets