Refactor GraphQL root `MergedObject` groups to reduce rustc query-depth overflow on Rust 1.94+

[kimhanbeom]

Problem

After upgrading Rust from 1.93.1 to 1.94.0, build/test can fail with:

error: queries overflow the depth limit!
= help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`minireview`)
= note: query depth increased by 130 when computing layout of `{async block@...QueryRoot<review_web::graphql::Query>...}`

The failure happens while compiling examples/minireview.rs (not from functional GraphQL logic changes).

Current root object shape

pub(super) struct Query(SubQueryOne, SubQueryTwo);
pub(super) struct Mutation(SubMutationOne, SubMutationTwo);

Each sub-object currently contains many merged query/mutation types, which increases derive expansion/type layout complexity.

Suspected cause (why this appears after 1.94.0)

• The issue is reproducible in our codebase on rustc 1.94.0 (4a4ef493e 2026-03-02), while 1.93.1 (01f6ddf75 2026-02-11) did not hit this limit.
• A likely contributing change is PR #149775, which added an extra wrapping layer in ManuallyDrop<T> layout paths (T -> MaybeDangling<T>).
• In async/type-heavy GraphQL root types, this additional layer appears to increase query-depth pressure and make overflow more likely on 1.94+.

Proposal

Refactor root GraphQL objects by splitting the merged groups into smaller units.

• Keep API/schema behavior unchanged.
• Only reorganize root merge structure.
• Avoid relying on #![recursion_limit = "..."] as the primary fix.

Example direction:

pub(super) struct Query(SubQueryOne, SubQueryTwo, SubQueryThree);
pub(super) struct Mutation(SubMutationOne, SubMutationTwo, SubMutationThree);

Acceptance Criteria

• cargo test --no-default-features --features auth-jwt --verbose passes without queries overflow the depth limit.
• cargo test --no-default-features --features auth-mtls --verbose passes without queries overflow the depth limit.

Notes

• This issue is about compile-time robustness across Rust versions (especially 1.94+), not feature changes.