Improve queries with in bulk

[cp-at-mit]

What are the relevant tickets?

NA

Description (What does it do?)

These are a few spots where we are performing database queries within loops such as:
for id in id_array:
Users.objects.get(id)

This PR updates this code to use in_bulk to get all of the objects by ID before looping through to interact with the objects. Hopefully this reduces the number of database queries in places.

How can this be tested?

Tests should pass, and functionality where the changes were made should not be impacted.

[github-actions]

OpenAPI Changes

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Unexpected changes? Ensure your branch is up-to-date with main (consider rebasing).

[sentry]

Bug: If the last order in a batch is missing from the database, accumulated association data is silently dropped and never sent to HubSpot due to a skipped final flush.
_{Severity: HIGH}

Suggested Fix

Decouple the final flush logic from the item processing loop. After the for loop concludes, add a final, unconditional check to flush any remaining items in contact_associations_batch and line_associations_batch. This ensures all accumulated data is sent, regardless of whether the last item was processed.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: hubspot_sync/tasks.py#L505-L507

Potential issue: In the `batch_upsert_associations_chunked` function, data is processed
in batches and flushed to HubSpot when the batch is full or on the last item. The code
checks for the last item using `idx == deal_count - 1`. However, if the last `order_id`
in the input list does not correspond to an existing `Order` in the database, a
`continue` statement is executed, skipping the rest of the loop body. This prevents the
final flush condition from ever being evaluated, causing any accumulated data in
`contact_associations_batch` and `line_associations_batch` to be silently dropped
instead of being sent to HubSpot.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[sentry]

Bug: The code calls .data on the result of super().to_representation(instance), which can be None if an order has no transactions, causing an AttributeError.
_{Severity: HIGH}

Suggested Fix

Before accessing the .data attribute on the transaction object, add a check to ensure it is not None. This handles cases where an order may not have any associated transactions.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: ecommerce/serializers/__init__.py#L725

Potential issue: In `TransactionPurchaseSerializer` and
`TransactionPurchaserSerializer`, the `to_representation` method calls
`super().to_representation(instance).data`. However, the parent method,
`TransactionDataSerializer.to_representation`, can return `None` if an order does not
have any associated transactions (e.g., for free courses, failed payments, or pending
orders). Calling `.data` on a `None` value will raise an `AttributeError`, causing a
crash. This will prevent users from viewing their order receipt for any order that lacks
a transaction.