SOLR-18034: Add index-time nested vector expansion tests

[arup-chauhan]

Description

This PR adds focused index-time test coverage for SOLR-18034 to validate how nested vector documents are constructed from multi-valued dense vectors.

The goal here is to make the existing behavior explicit and regression-safe before any broader lifecycle work.

Context from issue discussion

Issue: SOLR-18034

From the discussion, the scope is server-side indexing assistance for parent/child modeling of nested vectors (aligned with nested-vector direction from SOLR-17736, while avoiding overlap with broader multi-valued vector/query-time scope in SOLR-18074

This PR is intentionally scoped as PR-1: index-time behavior validation.

Changes

• Added test coverage to assert that a parent document with a multi-valued dense vector field is expanded into nested children (one vector per child) during update
  processing.
• Added assertions for generated child metadata and structure:
  • child id format
  • _nest_parent_
  • _nest_path_
  • parent field cleanup when expansion occurs
• Added complementary coverage to assert that a single-valued dense vector field remains on the parent and does not trigger nested-vector expansion.

Behavior / Safety

• No new runtime feature wiring or query behavior was introduced in this PR.
• Compatibility boundary:
  • Multi-valued dense vectors: existing expansion behavior is now explicitly verified by tests.
  • Single-valued dense vectors: existing fallback behavior remains unchanged (stays on parent, no generated nested vector children).
• Risk boundary is low: this is test/schema coverage only, intended to reduce regression risk for existing logic.

Validation

Executed and passing:

• ./gradlew :solr:core:test --tests org.apache.solr.update.TestNestedUpdateProcessor
• ./gradlew tidy
• ./gradlew check -x test

Result: all commands completed successfully for this branch.

Follow-ups

Planned next step (PR-2), based on maintainer feedback:

• lifecycle safety work (delete/update consistency for generated vector children),
• integration coverage for parent-child lifecycle behavior,
• small doc updates if behavior/user guidance needs clarification.

[arup-chauhan]

Hello everyone.

This PR is for the index-time part of SOLR-18034 (test coverage validating multivalued dense vector expansion into nested children)

Scope is intentionally narrow: this does not introduce new query-time behavior and keeps single-valued vector behavior unchanged, so risk is low, and boundaries stay aligned with related issues.

If this split looks good, I’ll proceed with the follow-up lifecycle-safety work in a second PR.
Also, I’ll add the changelog entry once this work is approved.