refactor(db-mongodb,drizzle): reduce race condition window between retrieving and using a session

[AlessioGr]

Description

Moves getSession (MongoDB) and getTransaction (Drizzle) calls to immediately before database operations.

Why

When sessions are retrieved early in a function but used much later, there's a race condition window: if a parallel operation commits/ends that session before we use it, we get MongoExpiredSessionError. By retrieving the session right before use, we minimize this window.

This alone significantly reduced transaction errors in #14651. By retrieving sessions closer to use, if a parallel operation commits the transaction before we retrieve it, we get undefined instead of an expired session. Operations then run without a session (which succeeds) rather than attempting to use an expired session (which errors).

Example:

// Before - 50+ line window where session could be ended by parallel operation
const session = await getSession(this, req)
const query = await buildQuery(...)     // Line 1
transform(data)                         // Line 2
// ... 48 more lines
await Model.find(query, { session })    // Line 50 - session might be expired!

// After - retrieve right before use
const query = await buildQuery(...)
transform(data)
const session = await getSession(this, req)  // Get it now
await Model.find(query, { session })         // Use immediately

[github-actions]

📦 esbuild Bundle Analysis for payload

This analysis was generated by esbuild-bundle-analyzer. 🤖
This PR introduced no changes to the esbuild bundle! 🙌

[DanRibbens]

I don't think this is right to do.

Looking at your simplified explanation:

// After - retrieve right before use
const query = await buildQuery(...)
transform(data)
const session = await getSession(this, req)  // Get it now
await Model.find(query, { session })         // Use immediately

We're saying that we can ignore changes that may have occurred on this transaction when querying related docs, then use the transaction for the find.

Happy to talk more about this and I think I could even make a test that will fail with this change.