Split API search route validation

[yvette-carlisle]

Summary

• Move API search payload/query validation helpers from routes/search.rs into routes/search/validation.rs.
• Keep route handlers and #[utoipa::path] OpenAPI annotations in search.rs.
• Preserve search create/raw/details validation semantics while reducing the parent route file size.

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo make lint-vstyle
• cargo make lint-rust
• cargo make test-rust (370 passed, 92 skipped)
• cargo make check (docs/fmt/check/clippy/vstyle/nextest; 370 passed, 92 skipped)
• cargo make test-rust-integration (92 passed, 370 skipped)
• cargo make check-trace-gate
• git diff --check
• git diff --cached --check
• Fresh read-only skeptic review of staged diff: pass, no objections

Cleanup

• Trace-gate Postgres was run in isolated elf-trace-gate compose project and removed with down -v --remove-orphans.
• Generated trace gate files were deleted before commit.