When a router has `featureFlagSearchExtension=true` and `featureFlagExtensionsInsideHybridSearch=false`, running an aggregation with a `$lookup` whose sub-pipeline contains `$search` crashes the router with `tassert(10978000)`.

The IFR kickback that should force this pipeline back to the legacy `$search` parser is placed on the post-desugar path (`DocumentSourceExtensionOptimizable::create()`). However, `$lookup` builds its sub-pipeline with `kOptionsMinimal` (`desugar=false`), so the pre-desugar wrapper `DocumentSourceExtensionForQueryShape` survives into wire dispatch — the exact state the tassert guards against. The same issue affects `$unionWith` sub-pipelines.

—

*Affected versions*

Regression introduced by `SERVER-117259` (`3d110ebc534c`, 2026-06-15), which lifted the pre-desugar wrapper's `LookupRequirement` from `kNotAllowed` to `kAllowed`, making the kickback in `SERVER-122433` unreachable on the `$lookup` sub-pipeline path.

—

*Steps to reproduce*

Start a sharded cluster with:
```
featureFlagSearchExtension: true
featureFlagExtensionsInsideHybridSearch: false
```
Run:
```js
db.coll.aggregate([{
$lookup: {
from: "coll",
pipeline: [{ $search: {} }, { $project:

}],
as: "matches"
}
}])
```
*Expected:* `ErrorCodes.SearchNotEnabled` (31082) — the router kicks back to the legacy parser, which fails because no mongot is running.
*Actual:* Router crashes with `tassert(10978000)` in `DocumentSourceExtensionForQueryShape::serialize`.

A self-contained reproducer exists at `jstests/noPassthrough/extensions/search_extension_in_lookup_sharded_wire_dispatch.js`.

—

*Root cause*

Two commits compounded the issue:

1. *SERVER-122433* placed the `featureFlagExtensionsInsideHybridSearch` kickback on `DocumentSourceExtensionOptimizable::create()` (post-desugar), with a TODO noting it would only be reachable once SERVER-117259 lifted the pre-desugar constraint.
2. *SERVER-117259* lifted that constraint and removed the TODO comment, without accounting for the fact that `DocumentSourceLookUp::initializeResolvedIntrospectionPipeline` builds sub-pipelines with `kOptionsMinimal` (`desugar=false`) — meaning the pre-desugar wrapper is never replaced by `DocumentSourceExtensionOptimizable`, and the kickback is never reached.

—

*Proposed fix*

Add the IFR kickback logic to the pre-desugar construction path in `expandableStageParamsToDocumentSourceFn` (`document_source_extension_optimizable.cpp`), before the `DocumentSourceExtensionForQueryShape` wrapper is constructed. Extract a shared helper (e.g., `throwHybridSearchKickbackForExtensionIfNecessary`) so the pre- and post-desugar sites stay in sync. This covers both `$lookup` and `$unionWith` sub-pipelines.

No changes needed to the router/shard retry loop — it is already exception-driven and will handle the kickback correctly.

Alternatives considered and rejected: flag-gating `constraints()` (no `ExpressionContext` access), lite-parse-time kickback (requires plumbing IFR context through `LiteParserOptions`), and forcing `kDesugarOnly` in `initializeResolvedIntrospectionPipeline` (high blast radius, doesn't fix `$unionWith`).

—

*Test plan*

• Add a new C++ unit test suite `DocumentSourceExtensionForQueryShapeInLookupKickbackTest` (symmetric with the existing `DocumentSourceExtensionOptimizableInLookupKickbackTest`) that constructs the pre-desugar wrapper via `expandableStageParamsToDocumentSourceFn` with `expCtx->setInLookup(true)` and asserts `IFRFlagRetry` is thrown.
• Confirm the existing multiversion test `jstests/multiVersion/genericBinVersion/ifr_search_extension_sharded_upgrade.js` passes (`searchInLookup` scenario now receives `SearchNotEnabled`).
• Run neighboring jstests to guard regressions: `search_ifr_flag_retry.js`, `extension_in_subpipeline_rejected.js`, `extension_in_lookup_flag_off.js`, `extension_in_lookup_with_views.js`.

—

*Related*

• *SERVER-117259* — regression source (lifted `LookupRequirement` on pre-desugar wrapper)
• *SERVER-122433* — introduced the kickback (in the wrong layer)
• *SPM-4488* — long-term plan to move to LiteParsed query shapes, which would eliminate the pre-desugar wrapper entirely