Fix and simplify ComputerUse sample

[SergeyMenshykh]

Simplifies the \Agent_Step15_ComputerUse\ sample to improve readability, reduces asset sizes, and expands the README with a detailed explanation of the simulation flow.

[Copilot]

Pull request overview

This PR updates the Agent_Step15_ComputerUse .NET sample to use AIProjectClient.AsAIAgent(...), expands the README to explain the simulated “computer use” state machine, and changes how screenshots are provided to the model (currently via uploaded OpenAI file IDs).

Changes:

• Refactors the sample program flow to use AIProjectClient.AsAIAgent(...) with a computer tool loop.
• Reworks screenshot handling utilities (upload/delete + mapping state transitions to screenshots).
• Expands README documentation with a state-transition table and interaction loop description.

Reviewed changes

Copilot reviewed 4 out of 10 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
dotnet/samples/02-agents/AgentsWithFoundry/Agent_Step15_ComputerUse/README.md	Adds a detailed explanation of the simulated computer-use loop and updates prerequisites/env var guidance.
dotnet/samples/02-agents/AgentsWithFoundry/Agent_Step15_ComputerUse/Program.cs	Refactors the sample to AsAIAgent(...) and implements a computer-action loop (currently using uploaded image file IDs).
dotnet/samples/02-agents/AgentsWithFoundry/Agent_Step15_ComputerUse/ComputerUseUtil.cs	Implements screenshot upload/delete and state-to-screenshot mapping for the loop.
dotnet/samples/02-agents/AgentsWithFoundry/Agent_Step15_ComputerUse/Agent_Step15_ComputerUse.csproj	Updates asset inclusion to .jpg screenshots copied to output.
dotnet/samples/02-agents/AgentsWithFoundry/Agent_Step15_ComputerUse/Assets/*.jpg	Adds/updates screenshot assets used by the simulation.