client-ofetch: no way to forward extra fetch options (allowlist in `buildOfetchOptions`)

[maxmaxme]

Description

buildOfetchOptions (in client-ofetch/bundle/utils.ts) builds the object passed to $ofetch.raw() from a fixed allowlist — it names each option and drops the rest (no ...opts spread). RequestOptions and the plugin config (only throwOnError) offer no escape hatch either.

So there's no supported way to forward an option the template doesn't list:

• Standard RequestInit fields ofetch supports (keepalive, mode, integrity, priority, …) get silently dropped.
• Custom options for a swapped-in fetch (e.g. onUploadProgress for an XHR-based fetch) can't reach it at all.

The only workaround today is string-patching the generated utils.gen.ts / types.gen.ts, which is brittle.

Possible fixes (any one works):

1. Spread standard RequestInit fields in buildOfetchOptions.
2. Plugin config allowlist, e.g. forwardOptions: ['keepalive', 'onUploadProgress'].
3. A fetchOptions passthrough on RequestOptions, merged verbatim into the $ofetch.raw() call.

Env: @hey-api/openapi-ts 0.99.0, @hey-api/client-ofetch, ofetch 2.x

[pullfrog]

The learnings file update looks clean. I added a focused "### ofetch option forwarding" subsection under "## Clients subsystem" with 4 bullets that capture the allowlist pattern, which fields are dropped, where config changes are needed, and where tests live.

Done.

Task list (3/5 completed)

• Read AGENTS.md and repo learnings to understand conventions
• Inspect client-ofetch bundle/utils.ts and related types/config
• Compare with other clients (fetch/axios) to identify existing pattern
• Draft implementation plan with chosen approach
• Post plan via pullfrog_report_progress

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