fix: handle BlobDelete event type in `batch_push_results`

[liammoat]

Purpose

• batch_push_results receives, but does not handle, blob delete events.
• This PR introduces the functionality to delete the search index, when a blob is deleted from Azure Storage

Does this introduce a breaking change?

• [ ] Yes
• [x] No

How to Test

1. Run the updated Azure Function
2. #1 Create a blob, uploading via Azure Storage
3. #2 Delete the blob from Azure Storage
4. #3 Trigger `BatchStartProcessing function

What to Check

Verify that the following are valid

1. #1 BatchPushResults is triggered, and the document is indexed.
2. #2 BatchPushResults is triggered, and the associated indexes are removed.
3. #3 BatchPushResults is triggered, as normal, and the documents are indexed.

Other Information

Remaining Actions

The following is outstanding for this PR:

• [ ] Tests

[github-actions[bot]]

Coverage Report •

File	Stmts	Miss	Cover	Missing
code/backend/batch
batch_push_results.py	36	0	100%
function_app.py	17	17	0%	1–8, 10, 12–14, 16, 19–22
code/backend/batch/utilities/search
search_handler_base.py	48	23	52%	13–15, 18–20, 24, 28, 32, 36, 40, 44, 48, 51–52, 54–56, 58–59, 62–63, 65
TOTAL	2424	684	71%

Tests	Skipped	Failures	Errors	Time
201	0 :zzz:	0 :x:	0 :fire:	12.926s :stopwatch:

[liammoat]

Team - Couple of things to note about this PR:

1. There is likely a better way to do this - however, I have tried to implement it with as few downstream changes as possible (i.e. I don't want to refactor everything to fix this)
2. With this change in mind, I propose the admin app should delete the blob, and let the function handle the index.
3. I will leave this in draft until I have introduce some tests to cover the core behaviour

[superhindupur]

Team - Couple of things to note about this PR:

1. There is likely a better way to do this - however, I have tried to implement it with as few downstream changes as possible (i.e. I don't want to refactor everything to fix this)
2. With this change in mind, I propose the admin app should delete the blob, and let the function handle the index.
3. I will leave this in draft until I have introduce some tests to cover the core behaviour

Definitely agree with " I propose the admin app should delete the blob, and let the function handle the index." - could you please create an issue to cover this if there isn't one already?

Update: I see that @komalg1 is working on this in #876 - would be worth the two of you catching up.

[superhindupur]

@liammoat thanks so much for this, really appreciate it! ❤️