Strengthen test assertions by explicitness

[StephanMeijer]

• Replace partial/lazy test assertions with explicit assert response.json() == {...} checks
• Split multi-scenario tests into focused single-scenario tests

[Copilot]

Pull request overview

Tightens backend test expectations around document bulk indexing/deletion APIs and admin selftest redirects by replacing partial assertions with explicit full-response comparisons and splitting multi-scenario tests into focused cases.

Changes:

• Strengthen API tests by asserting exact response.json() payloads for bulk indexing and deletion endpoints.
• Split “invalid params” coverage in document deletion tests into separate single-scenario tests.
• Make admin selftest redirect assertions more explicit by checking the full redirect URL.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/backend/core/tests/test_api_documents_index_bulk.py	Replaces partial status-only assertions with explicit full response payload equality checks.
src/backend/core/tests/test_api_documents_delete.py	Makes delete API assertions fully explicit and splits invalid-parameter scenarios into multiple tests.
src/backend/core/tests/test_admin_selftest.py	Updates redirect assertions to check the full expected login redirect URL.

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