Snapshot example services' GraphQL schemas as insta tests (backport of #6542)

[ma2bd]

Motivation

Backport of #6542 to testnet_conway.

We record each application's serde/BCS formats as an insta snapshot (via BcsApplication) so that unintended changes to operation/message/event encodings show up as a reviewable diff. This adds the equivalent guard for the GraphQL schema exposed by each application's service.

Proposal

For every example that implements BcsApplication and exposes a GraphQL service, add a #[cfg(test)] schema_sdl test in service.rs that snapshots the service's GraphQL SDL. Most services extract a private fn schema(&self) -> Schema<…> and have handle_query delegate to it, so the snapshotted SDL cannot drift from what is served; the test builds the service against a MockServiceRuntime and snapshots schema().sdl().

The snapshot tests are gated #[cfg(not(target_arch = "wasm32"))] (mirroring the existing tests/format.rs convention), because insta is a non-wasm dev-dependency and the example services are also compiled for wasm32 with --all-targets.

Notes specific to this backport

This is not a clean cherry-pick of #6542 — testnet_conway differs from main in several examples, so the following were adapted and the affected SDL snapshots regenerated against testnet_conway:

• counter keeps its tutorial // ANCHOR markers and QueryRoot/MutationRoot structs, so its handle_query is left intact and the test builds the schema directly (in a separate gated schema_tests module).
• native-fungible uses NativeFungibleOperation here (vs FungibleOperation on main).
• matching-engine loads its state via root_view_storage_context() here (its Parameters has no price_decimals field), so the test uses the simple load path.
• fungible imports OwnerSpender from a different module here.
• wrapped-fungible gained an insta non-wasm dev-dependency (it was already present on the other examples).
• task-processor gained a wasmer-featured linera-sdk dev-dependency (needed by the mock ServiceRuntime).
• llm can't construct its service in a unit test (new() downloads model weights), so its test builds the schema directly; this does not affect the SDL.

Test Plan

• cargo test -p <example> --bin <example>_service schema_sdl passes for all 16 services; re-running without INSTA_UPDATE confirms the SDL is deterministic.
• cargo fmt -- --check (pinned nightly) is clean across the examples workspace.
• cargo clippy --all-targets --all-features --target wasm32-unknown-unknown compiles (the schema tests are excluded from wasm).
• Note: on aarch64 hosts, gen-nft and llm need RUSTFLAGS=-C target-feature=+fullfp16 to build candle; irrelevant on x86 CI.

Release Plan

• These changes follow the usual release cycle.

Links

• Forward port: Snapshot example services' GraphQL schemas as insta tests #6542
• reviewer checklist