Add Agents API and September 2026 API updates

[mkht]

Summary

• add OpenAI Agents API public beta support for reusable agents, environments, sessions/events/items/turns/subagents/artifacts, vaults, and credentials
• add GPT Image 2.5 Sunburst/Flare model support, xhigh/max quality, and arbitrary supported image sizes
• add prompt cache diagnostics comparison and prewarming options
• add secure MCP tunnel_id support while retaining deprecated connector_id compatibility
• avoid parsing commentary-phase response text as Structured Output JSON
• update command docs, README, manifest exports, and generated external help

Webhook endpoint management and GPT-Live are intentionally out of scope for this PR.

Validation

• Invoke-Pester -Tag Offline on the six affected test files: 75 passed, 0 failed
• PSScriptAnalyzer with PSScriptAnalyzerRules.psd1: 0 findings for changed implementation files
• PowerShell syntax parsing: all new and changed scripts passed
• manifest exports, README documentation links, CRLF line endings, and cached diff checks passed

[mkht]

This combined PR has been split to reduce review scope:

• Add September 2026 API compatibility updates #51 — September 2026 P0 API compatibility updates
• Add OpenAI Agents API support #52 — OpenAI Agents API support

Both replacement PRs target v5. Closing this PR as superseded.