[Search 2026-04-01] java SDK codegen

[Copilot]

Regenerates azure-search-documents from spec commit 18ebae7ea8f6aefd83268aacdb1f480b24842726 (API 2026-04-01, stable) and prepares the 12.1.0 release, based off and targeting release/azure-search-documents_12.1.0.

Regenerating against the new spec commit yields byte-for-byte identical generated code to the released 12.0.0 — the two commits differ only outside the Search 2026-04-01 data-plane surface. SearchCustomizations.java applies cleanly with no version/enum reconciliation needed (getLatest() already returns 2026-04-01). This PR is therefore effectively a spec-pin refresh plus version metadata.

Changes

• Spec pin — tsp-location.yaml commit → 18ebae7ea8f6aefd83268aacdb1f480b24842726.
• Version — 12.0.0 → 12.1.0 across pom.xml, README.md, src/samples/README.md, and eng/versioning/version_client.txt; dependent sdk/search/azure-search-perf/pom.xml bumped to keep the {x-version-update;...;current} markers consistent (enforced by pom_file_version_scanner.ps1).
• CHANGELOG — new ## 12.1.0 (Unreleased) section noting the regeneration with no public API changes; 12.0.0 section preserved.

Notes for reviewers

• No source/test changes: the regenerated surface is unchanged, so no new tests were warranted.
• eng/versioning/version_client.txt and azure-search-perf/pom.xml fall just outside azure-search-documents/** but are the mandatory mechanical consequence of the version bump.

[azure-pipelines]

Azure Pipelines:
35 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.