test: add C FFI integration tests

[mattisonchao]

Summary

• Add 7 C integration tests that exercise the full FFI API against a real Oxia server
  • test_put_get: put a key, get it back, verify value
  • test_get_not_found: verify KeyNotFound error for missing key
  • test_delete: put, delete, verify gone
  • test_list: put multiple keys, list range, verify sorted order
  • test_delete_range: put keys, delete range, verify all removed
  • test_put_overwrite: put twice, verify version increases
  • test_binary_value: put/get binary (non-UTF8) data
• Rust test harness starts Oxia container via testcontainers, compiles the C test with cc, and runs it
• Add rlib crate type to liboxia-ffi to support integration tests

Test plan

• All 7 C FFI tests pass locally
• CI passes (fmt, clippy, build, tests)