Make Execution API secret reads async

[Dev-iL]

related: #67799, #72329

Execution API Variable and Connection reads previously depended on synchronous core resolvers, preventing the server from using native async backend I/O. This change adds async server-side resolution while preserving the existing secrets backend configuration, ordering, and behavior:

• Adds optional aget_variable and aget_connection dispatch.
• Uses native async SQLAlchemy reads for the metastore backend.
• Runs unchanged synchronous backends, cache access, masking, and connection processing in bounded worker threads.
• Preserves custom sync overrides, team scope, cache semantics, access-denial behavior, cancellation, and existing HTTP responses.
• Migrates the Variable and Connection GET routes to await the new resolver.
• Disposes the test async engine inside each TestClient event loop, preventing pooled MySQL connections from crossing loop lifetimes.
• Documents async backend adoption, fallback precedence, resource ownership, cancellation limits, and rollback.

Native cloud-provider clients remain follow-up work, separate from the SDK Variable API work in #72329.

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