Pin a DB connection for the Session's lifetime (with_connection) to survive implicit check-ins

[sncalvo]

Problem

Rails 7.2+ changed connection checkout semantics (per-request check-in via the executor). A Session that doesn't pin a connection can have its connection returned to the pool mid-block, which invalidates temp tables on Postgres.

Proposal

Wrap the session body in ActiveRecord::Base.connection_pool.with_connection { ... } (or the source_model's pool), pinning the connection for the duration of the block.

StagingTable.stage(User) do |staging|  # automatic with_connection
  staging.insert(rows)
end

Opt-out for advanced users who manage their own connection lifecycle:

StagingTable.stage(User, pin_connection: false) { ... }

Acceptance criteria

• Default behavior pins the connection for the stage { } block
• Manual-control API gets a Session#with_pinned_connection equivalent
• Test: under a forced pool-check-in mid-block (simulate via executor), temp table survives
• README note on Rails 7.2+ behavior change and why pinning matters