Fix resolver session to create independent sessions

[shangyian]

Summary

The previous fix (#2024) replaced info.context["session"] with resolver_session() in all GraphQL resolvers, but resolver_session delegated to session_context() which returns request.state.test_session and is set by get_context on every request, not just for tests. This meant all resolvers still shared one session in production, and the concurrent-session crash persisted.

This now calls get_session() directly in production, only reusing the shared session in tests (detected via dependency_overrides).

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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