[WIP] Update documentation for openai assistants agent

[Copilot]

• Explore repo and understand current state of dotnet-microsoft-agent-framework skill
• Update references/official-docs/user-guide/agents/agent-types/openai-assistants-agent.md:
  • Change title from "OpenAI Assistants Agents" to "OpenAI Agents" (matches new upstream title)
  • Update ms.date to 03/17/2026
  • Strengthen deprecation warning with shutdown date (August 26, 2026) and migration path
• Update references/providers.md: mark OpenAI Assistants with deprecation/shutdown date
• Bump SKILL.md version to 1.5.0 to reflect the material guidance change
• Run catalog validation
• Code review and CodeQL check

Original prompt

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This section details on the original issue you should resolve

<issue_title>Upstream update: learn.microsoft.comagent-framework/user-guide/agents/agent-types/openai-assistants-agent documentation documentation changed</issue_title>
<issue_description>Automation detected an upstream change for learn.microsoft.comagent-framework/user-guide/agents/agent-types/openai-assistants-agent documentation.

• Watch id: agent-framework-docs-user-guide-agents-agent-types-openai-assistants-agent
• Kind: http_document
• Source: https://learn.microsoft.com/agent-framework/user-guide/agents/agent-types/openai-assistants-agent
• Current value: "oxiPQfONsMxzmCpHl/1jFF3R9Hk="
• Previous value: "qM38lXAhvR7khQuCA43hZmVdXac="

Observed detail:

• OpenAI Agents | Microsoft Learn | ETag "oxiPQfONsMxzmCpHl/1jFF3R9Hk="

Why this matters:

• Review the linked skills when https://learn.microsoft.com/agent-framework/user-guide/agents/agent-types/openai-assistants-agent changes.

Likely skills to review:

• dotnet-microsoft-agent-framework

Suggested follow-up:

• Review the upstream release notes or documentation diff
• Update the affected files under skills/
• Update README.md if framework coverage or guidance changed
• Close this issue after the catalog has been refreshed

<agent_instructions>Review all tasks and if needed make jsut one pr an done branch with all updates for skills and docs </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Upstream update: learn.microsoft.comagent-framework/user-guide/agents/agent-types/openai-assistants-agent documentation documentation changed #15

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