Summary

An extension $vectorSearch inside a $lookup subpipeline that targets a view fails with Location40602 ("$_extensionVectorSearch is only valid as the first stage in a pipeline"): view resolution prepends the view's transform stages into the subpipeline, pushing the desugared $_extensionVectorSearch out of first position.

Description

When $lookup.from is a view and the subpipeline begins with an extension $vectorSearch, view resolution prepends the view's stages ahead of it. The extension stage desugars to $_extensionVectorSearch, which requires being the first stage, so the query fails:

code: 40602, codeName: "Location40602"
errmsg: "$\_extensionVectorSearch is only valid as the first stage in a pipeline"

Example failing pipeline:

db..aggregate\(\[{
  $lookup: {
    from: "",   // e.g. moviesWithEnrichedTitle
    pipeline: \[{$vectorSearch: {queryVector: \[...\], path: "...", index: "...", ...}}\],
    ...
  }
}\]\)

With the flag off, the IFR kickback retries with the legacy $vectorSearch, which handles the view prepend; with the flag on, the extension stage cannot tolerate stages ahead of it.

Environment

* Suites: with_mongot_extension_single_node.
* Flag: featureFlagExtensionsInsideHybridSearch: true.

Failing tests (currently excluded)

* jstests/with_mongot/e2e/views/vector_search/lookup_vector_search_view.js
* jstests/with_mongot/e2e/views/vector_search/lookup_subpipeline_combinations.js
* jstests/with_mongot/e2e/views/vector_search/lookup_nested_view.js
* jstests/with_mongot/e2e/views/vector_search/lookup_identity_and_combined.js

Acceptance criteria

Remove the exclusion for these four tests from buildscripts/resmokeconfig/matrix_suites/overrides/with_mongot_extension.yml (regenerate matrix suites) and they pass in the with_mongot_extension_* suites with the flag on.