Fix partial application generalization for ...

[cknitt]

Fix partial application generalization for ...

Summary

Fixes #8334.

ReScript treated foo(arg, ...) differently from foo(arg, _) during
generalization:

• foo(arg, _) was parsed into an explicit lambda, so it was treated as a
  function value and could be generalized.
• foo(arg, ...) stayed as a partial application node in the typed tree, so it
  was treated as expansive and could end up with weak type variables.

That caused code like this to fail:

type hook

external hook: hook = "hook"
external addHook: (hook, 'a => unit) => unit = "addHook"

let addHookPartial = addHook(hook, ...)

with:

This expression's type contains type variables that cannot be generalized:
('_weak1 => unit) => unit

Fix

Treat ReScript partial applications with partial = true as nonexpansive in
the value restriction check.

This matches the runtime/codegen behavior: foo(arg, ...) is ultimately lowered
to a wrapper function, so it should be classified like an explicit lambda for
generalization purposes.

Tests

Added a regression to tests/tests/src/PartialApplicationNoRuntimeCurry.res
covering the reported polymorphic callback case:

• create addHookPartial = addHook(hook, ...)
• use it at int
• use it again at string

Updated the expected generated output in the matching .mjs fixture.

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commit: 55f684a