feat(openai): add file search tool support

[dancer]

background

users want to ask questions about their documents without building complex search systems. OpenAI's file search handles this on their servers.

summary

• add openai file search tool with document sources
• implement streaming file citations with real-time source extraction

verification

• all tests pass including file search streaming test
• document sources created with proper metadata and MIME types
• examples demonstrate file search integration

tasks

• fileSearch tool added to openai-tools.ts
• file citation processing in language model
• document source creation with metadata
• streaming support with source extraction
• test coverage and examples

[lgrammel]

Looks good. Pls updated the openai provider docs page.