docs: add gpt-image-1.5 model support for OpenAI image operations

[alexsorokoletov]

Documentation update for this PR: n8n-io/n8n#23375

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Summary by cubic

Adds gpt-image-1.5 support to OpenAI Image operations docs, covering generation and editing with updated quality, resolution, and output settings.

• New Features
  • Generate an Image: model list now includes gpt-image-1.5; added quality (Auto/High/Medium/Low) and resolution options for gpt-image-1/-1.5.
  • Edit Image: supports gpt-image-1.5; quality, output format (png/webp/jpg), compression, background, and input fidelity now available for gpt-image-1/-1.5.

^{Written for commit 6314244. Summary will update automatically on new commits.}

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[cubic-dev-ai]

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