Generate an image from a text prompt.

Returns a consistent content array for both stdio and SSE transports:
1. TextContent: Enhanced metadata including generation info and file details
2. ResourceLink: Main image file reference with context-appropriate URI:
   - SSE: Absolute URL built from request context (X-Forwarded-* headers), ZIMAGE_BASE_URL, or relative path
   - Stdio: file:// URI for local access
3. ImageContent: Thumbnail preview (base64 PNG, max 400px)

URI Building Priority (SSE):
1. Context parameter (ctx.request_context.request) - builds absolute URL from request headers
2. ZIMAGE_BASE_URL environment variable - uses configured base URL
3. Relative URL - fallback when no other method available

File metadata (filename, file_path) is in TextContent to avoid duplication in ResourceLink.

For long-running operations (high steps/large images), this function will:
- Send progress notifications at key milestones via ctx.report_progress()
- Handle client disconnections gracefully