Apps review 2

[Rigidity]

Note

Medium Risk
Touches bridge dispatch, approval resolution, and wallet/secret bridge methods; behavior change if anything relied on approving non-user pending requests or the old dynamic registry path.

Overview
This PR refactors the Sage apps bridge and related host code: it drops async-trait / erased-serde, replaces dynamic BridgeMethod trait objects with typed user/system registries and BridgeMethodHandler, and standardizes handler results as serde_json::Value via bridge_result.

Bridge approval resolution no longer re-enters the generic shared executor for approved calls; it only runs user-bridge methods through UserBridgeRegistry (execute_approved_user_bridge_request), and rejects resolving non-user pending approvals.

App lifecycle mutations use AsyncFnOnce instead of Box::pin futures; install sources use native impl Future traits. Sandbox modules are re-exported at the crate root with broader pub(crate) probe APIs, and several import paths are flattened.

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