fix: preserve structuredDocumentInfo/unstructuredDocumentInfo references in GoogleConversationalSearchTracer

[viniciusdsmello]

Summary

• GoogleConversationalSearchTracer#extract_references only handled the chunk_info variant of Answer.Reference.content (a 3-way protobuf oneof), silently dropping references that use structured_document_info or unstructured_document_info via next nil if chunk_info.nil? + .compact.
• Dispatches over all three content oneof variants now. structured_document_info maps document/uri/title/struct_data; unstructured_document_info maps document/uri/title plus content joined from its chunk_contents.
• As a side effect, this also fixes the internal inconsistency where metadata[:references_count] (raw count) could disagree with the actually-traced references (previously silently truncated).

Fixes #35

Why traced_context correctly stays 0 for structured docs

A structured_document_info reference carries no free-text — only structured JSON (struct_data) — so it populates references but intentionally contributes nothing to the RAG context array. That's not a partial fix; there's no prose to surface. struct_data itself is preserved on the reference.

Verification

• Reproduced the issue's exact offline repro against main before the fix (traced_references: 0) and confirmed it now returns traced_references: 1.
• Cross-checked field mappings against the upstream proto (googleapis/googleapis: google/cloud/discoveryengine/v1/answer.proto) — StructuredDocumentInfo fields are exactly document, struct_data, title, uri.
• Added regression tests via TDD (one RED→GREEN cycle per variant): chunk_info (characterizes pre-existing behavior), structured_document_info, unstructured_document_info, mixed variants, no-known-variant (doesn't raise), and references_count metadata consistency.
• rubocop/steep/srb typecheck all clean on the touched files (3 pre-existing Layout/LineLength offenses elsewhere in the file are untouched by this change).

Test plan

• bundle exec rake test TEST=test/openlayer/integrations/google_conversational_search_tracer_test.rb — 6 runs, 21 assertions, 0 failures
• bundle exec rake test (full suite) — same pre-existing 31 mock-server connection errors as on main (documented in CONTRIBUTING.md as needing ./scripts/mock), 0 new failures
• bundle exec rake lint — clean