Fix topic page pagination

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/12397

Description (What does it do?)

This PR fixes an issue where the topic pages show more pages than can be paginated to. This is due to the use of exact=False for the final count returned by the api which ends up showing more results than there actually are.

How can this be tested?

1. checkout main - make sure you have embeddings locally
2. go to a top-level topic page such as http://open.odl.local:8062/c/topic/engineering -> click on the very last page of results and it should come up empty.
3. checkout this branch and attempt the same - it should never be empty and the page count should be accurate

[github-actions]

OpenAPI Changes

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[Copilot]

Pull request overview

This PR updates the vector search count logic used by topic-page pagination so the UI’s total-results count is exact, preventing “extra” pages that render empty results.

Changes:

• Switch Qdrant count calls used for pagination totals from approximate (exact=False) to exact (exact=True).
• Add a regression test ensuring the vector search endpoint requests an exact count from Qdrant.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
vector_search/views.py	Uses an exact Qdrant count for paginator totals to avoid advertising empty pages.
vector_search/views_test.py	Adds a test that asserts the Qdrant count call is made with exact=True.