fix: send createSession 201 only after the transaction commits

[ionfwsrijan]

Problem

createSession (backend/controllers/sessionController.js) runs all writes inside mongoSession.withTransaction(async () => { ... }), but:

• Lines 95–98: res.status(201).json(...) is sent inside the callback — before Mongoose commits. If the commit subsequently fails (network error, replica-set failover, duplicate key on a concurrent write), the client already got a 201 + full session object while the transaction was rolled back: a phantom success.
• Lines 39–57: validation failures return res.status(400) inside the callback. Mongoose's withTransaction commits based on the callback's return value, so returning an Express response object commits instead of aborting — the opposite of the code's obvious intent.

Fix

• Validate before opening the transaction — role/experience/MAX_SESSIONS checks now run first and return 4xx from the outer handler, never from inside the callback.
• Move res.* out of the callback — the callback returns the created session, and res.status(201).json(...) is sent only after withTransaction resolves (i.e. after commit).
• The SESSION_LIMIT_REACHED throw is replaced by an early 400 return ahead of the transaction.
• endSession() stays in finally, guarded for the pre-transaction validation paths.

Files changed

• backend/controllers/sessionController.js — restructured createSession.
• backend/tests/sessionController.create.transaction.unit.test.js — tests for 400s without starting a transaction, no 201 on commit failure, and 201 only after commit.

Testing

npx vitest run tests/sessionController.create.transaction.unit.test.js — 5/5 passing.

Closes #1442

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📥 Commits

Reviewing files that changed from the base of the PR and between 8acb5b8 and 830c85c.

📒 Files selected for processing (2)

• backend/controllers/sessionController.js
• backend/tests/sessionController.create.transaction.unit.test.js

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