fix(graphql): prevent value fields from shadowing response input

[amnn]

Description

The Response derive generated one local binding per response field. When a field named value came before another field, its binding shadowed the value parameter containing the original JSON response, so later fields tried to traverse the extracted field instead of the response root, which often led to compilation (type) errors.

Before:

pub fn from_value(value: serde_json::Value) -> Result<Self, String> {
    let value = {
        let current = &value;
        // extract value
    };
    let digest = {
        let current = &value;
        // incorrectly extracts from the value field
    };
    Ok(Self { value, digest })
}

After:

pub fn from_value(value: serde_json::Value) -> Result<Self, String> {
    Ok(Self {
        value: {
            let current = &value;
            // extract value
        },
        digest: {
            let current = &value;
            // extracts from the original response
        },
    })
}

This change initializes fields directly in the struct literal so field names do not introduce local bindings.

Test Plan

Adds a regression test with a value field followed by another extraction.

$ cargo nextest run -p sui-graphql-macros --test extraction_test \
  -- test_field_named_value_does_not_shadow_response_value

[tpham-mysten]

Great catch! Thanks @amnn!