docs: clarify custom fetch behavior for custom CA bundles

[raashish1601]

Refs #1758.

Summary

• clarify that fetchOptions are forwarded to the active fetch implementation unchanged, so Undici-specific fields like dispatcher depend on the underlying runtime fetch
• document a custom undici.fetch workaround for Node runtimes that patch globalThis.fetch and need NODE_EXTRA_CA_CERTS / custom CA support
• add a focused regression proving client-level and request-level fetchOptions are passed through to a custom fetch implementation

Testing

• corepack yarn run jest tests/index.test.ts --runInBand -t "passes merged fetchOptions through to custom fetch|custom fetch|normalized method"
• corepack yarn run prettier --check README.md src/client.ts tests/index.test.ts
• corepack yarn run eslint src/client.ts tests/index.test.ts

[raashish1601]

Closing this duplicate in favor of #1803. The validated passthrough/docs follow-up from this branch was pushed onto raashish1601:contributor-53/openai-node-1758 as commit 693bf01b.