Ready patch: accept both snippet status key styles in Vertex AI Search client

[Gujiassh]

I prepared a small fix for search/cloud-function/python/vertex_ai_search_client.py that accepts both snippetStatus and snippet_status when parsing snippet-only document results.

The branch is already pushed here:

• compare view: main...Gujiassh:fix/snippet-status-key-compat
• fork branch: Gujiassh:fix/snippet-status-key-compat

Why this matters:

• some payloads use snake_case snippet_status
• the current parser only accepts camelCase snippetStatus
• when that happens, snippet-only results get silently dropped even though the snippet succeeded

Patch contents:

• accept both key styles in _parse_snippets
• add a regression test covering both variants in test_vertex_ai_search_client.py

Local verification on the branch:

• search/cloud-function/python/.venv/bin/python -m pytest test_vertex_ai_search_client.py -k snippet_only_accepts_snippet_status_variants -q
• search/cloud-function/python/.venv/bin/python -m pytest test_vertex_ai_search_client.py -q

I wasn't able to open a PR directly because GitHub rejected CreatePullRequest for this repository from my account, but the branch and compare URL are ready if a maintainer wants to cherry-pick or reopen it as a PR.

[Gujiassh]

Follow-up: the compare view is still live and the branch remains valid, but direct PR creation from my fork continues to be rejected by repository permissions. I am leaving this issue open as the maintainer-visible handoff point for the ready patch.

[Gujiassh]

Follow-up refresh on 2026-04-05 JST: the branch Gujiassh:fix/snippet-status-key-compat is still current against main and the local verification was rerun successfully today. Validation: cd search/cloud-function/python && ./.venv/bin/python -m pytest test_vertex_ai_search_client.py -k snippet_only_accepts_snippet_status_variants -q (2 passed, 10 deselected) and cd search/cloud-function/python && ./.venv/bin/python -m pytest test_vertex_ai_search_client.py -q (12 passed). Direct PR creation from my account is still blocked by repository permissions, so this issue remains the maintainer-visible handoff point for the ready patch: main...Gujiassh:fix/snippet-status-key-compat