Fix search (and vector search) index errors

[jamis]

Fixes a few errors in tests with search and vector search tests; also fixes an issue with vector_search and auto_embed_search due to filter being used instead of a separate $match stage.

[Copilot]

Pull request overview

This PR addresses failing search/vector-search specs and adjusts instance-level vector search query construction to avoid using $vectorSearch.filter for “exclude self”, instead applying self-exclusion via a separate $match stage.

Changes:

• Extend test client configuration to pass direct_connection when provided by the URI.
• Make search-index specs more robust against Atlas-normalized index definitions and index ordering differences.
• Update #vector_search / #auto_embed_search instance pipelines to exclude the source document post-search and add targeted unit tests for the constructed aggregation pipeline.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
spec/spec_helper.rb	Passes direct_connection from the configured MongoDB URI into the test client options.
spec/mongoid/search_indexable_spec.rb	Adds pipeline-construction unit tests; makes Atlas integration expectations more tolerant of server normalization and ordering.
lib/mongoid/search_indexable.rb	Refactors instance-level vector search to use a post-search $match for self-exclusion and over-fetches by 1 to avoid short counts.

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[comandeo-mongo]

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