fix: sanitize empty base64 image payloads for OpenAI requests

[YanzheL]

Summary

• Skips empty or whitespace-only base64 image URLs in the Chat Completions compatibility translator before they are processed as input_image parts.
• Sanitizes native OpenAI Responses payloads by removing empty base64 input_image entries from both nested message content and top-level input items.
• Adds regression tests covering exact-empty, whitespace-only, nested, and top-level image payload cases while ensuring valid image inputs are preserved.

Testing

• go test ./internal/pkg/apicompat
• go test ./internal/service -run 'TestSanitizeEmptyBase64InputImagesInOpenAIRequestBodyMap|TestSanitizeEmptyBase64InputImagesInOpenAIBody|TestExtractOpenAIReasoningEffortFromBody|TestGetOpenAIRequestBodyMap_UsesContextCache|TestGetOpenAIRequestBodyMap_ParseErrorWithoutCache|TestGetOpenAIRequestBodyMap_WriteBackContextCache'
• go build ./cmd/server

Closes #1011
Closes #1014