Fix unpacking first-class module in default argument of react component

[cknitt]

Fixes #7917

PR Description: Fix module unpacking in default React props

Summary

This change fixes React component arguments that both:

• use a default value, and
• destructure a first-class module in the parameter pattern

The motivating case is:

module M = {
  @react.component
  let make = () => React.null
}

module type S = module type of M

@react.component
let make = (~component as module(C: S)=module(M: S)) => <C />

Before this change, the JSX PPX lowered defaulted props in a way that only worked reliably for simple variable patterns. First-class module unpacking patterns need to happen after the optional prop has already been resolved to a plain value.

Problem

For defaulted props, the PPX needs to transform:

~component as module(C: S)=...

into the internal props-record form used by React components.

The old lowering coupled optional-prop handling too closely to the original pattern shape. That works for identifiers, but not for module-unpack patterns, because unpacking module(C: S) must happen in a standalone let pattern over a resolved value, not inside the option-handling step.

Fix

The fix makes defaulted props go through a uniform two-step lowering:

1. Bind the prop record field to a temporary __<label>.
2. Resolve the optional prop to __<label>_value.
3. Destructure the original pattern from that resolved value in a separate let.

Conceptually, the generated code now looks like:

let __component_value = switch __component {
| Some(component) => component
| None => module(M: S)
}

let module(C: S) = __component_value
<C />

This preserves the existing behavior for simple defaulted props while making first-class module unpacking work correctly.

Tests

The change is covered at two levels:

• syntax snapshot coverage in
  tests/syntax_tests/data/ppx/react/defaultPatternProp.res
• end-to-end build coverage in
  tests/build_tests/react_ppx/src/issue_7917_test.res

Why this shape

This approach is simpler than special-casing only variable-like patterns:

• it uses the same lowering strategy for all defaulted props
• it keeps the record-pattern binding phase and the destructuring phase separate
• it naturally supports nontrivial patterns such as first-class module unpacking

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commit: 4fe7802

[cristianoc]

This changes generated code for non-first-class-module cases, but seems legit to be consistent.

[cknitt]

This changes generated code for non-first-class-module cases, but seems legit to be consistent.

Yes, I did have a version with more special cases before, but didn't quite like it.