Add more embedding providers and create latex

[Fisher-007]

This PR adds a Story-to-LaTeX conversion script, improves pipeline for more embedding providers. Tested normally with no new errors reported.

Changes

1. Story to LaTeX (Paper-KG-Pipeline/scripts/story_to_latex.py)

• New: Script to convert final_story.json into arXiv-style LaTeX papers
• Behavior: Reads story from a results directory, fills an arXiv template, and outputs paper.tex

2. Additional Embedding Providers (Paper-KG-Pipeline/src/idea2paper/infra/embeddings.py/startup_preflight.py)

• New: Support for Gemini native embedding API (EMBEDDING_PROVIDER=gemini)
• Existing: OpenAI-compatible APIs remain the default (EMBEDDING_PROVIDER=openai_compatible)

3. README update

• Outputs: Document paper.tex as an output artifact
• Story to LaTeX: Add usage, examples, and option descriptions
• Embedding: Document Gemini support via EMBEDDING_PROVIDER=gemini

4. .env.example update

• Add EMBEDDING_PROVIDER (openai_compatible | gemini)
• Add configuration examples for both OpenAI-compatible and Gemini