v1.0.1: add ./providers subpath export

[valentinozegna]

Summary

• Add ./providers subpath export to package.json so external consumers can import provider configs directly (e.g., import { googleProvider } from "@intelligentelectron/pdf-analyzer/providers")
• New barrel file src/providers/index.ts re-exports all provider configs, registry utilities, isGeminiFileUri, and provider types

Test plan

• npm run type-check passes
• npm run lint passes
• npm test passes (44 passed, 2 skipped)