Fix concurrent session crash in GraphQL multi-field queries

[shangyian]

Summary

GraphQL resolvers shared a single AsyncSession via info.context["session"]. Strawberry resolves top-level fields concurrently, so a query like { findNodes(...) searchTags(...) } would fire two await session.execute() calls on the same session simultaneously, causing an InvalidCachedStatementError like "concurrent operations are not permitted". This was a latent bug in all resolvers and could be triggered by a client sending multiple top-level fields in one request.

Each resolver now creates its own session via resolver_session(info), a new helper in that wraps session_context(request). DataLoaders already used independent sessions and are unaffected.

Test Plan

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

Deployment Plan

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