Add OpenAI provider support to Invoke-PSClaudeCode

[dfinke]

Motivation

• Support LLMs beyond Anthropic so the agent can use OpenAI-compatible models and function-calling flows.
• Make provider selection and API key handling explicit to allow swapping providers without branching code changes.

Description

• Added a -Provider parameter with ValidateSet("Anthropic","OpenAI") and provider-aware API key resolution via Get-ApiKey.
• Implemented Convert-ToolsForProvider, Normalize-Response, Invoke-ModelRequest, and Append-ToolResults to translate tool definitions, call the correct API endpoints, and normalize responses between Anthropic and OpenAI formats.
• Updated the agent loop and Run-SubAgent to use Invoke-ModelRequest and provider-specific tool formats, and adjusted tool-result marshalling so results are appended correctly for each provider.
• Updated comment-based help and README.md to document the new -Provider option and environment variables (ANTHROPIC_API_KEY or OPENAI_API_KEY) with example usage.

Testing

• No automated tests were executed for this change.

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Codex Task