fix: prevent backend segfault from user-mapping FFI and harden the FDW

A scan running under a role with no user mapping could crash the backend
with SIGSEGV (signal 11). In new(), GetUserMapping() raises a Postgres
ERROR (a C longjmp) when no mapping exists; this unwound past the already
constructed multi-threaded tokio runtime, skipping its Drop and leaking
worker threads until the backend destabilized. pg_timetable's repeated
scheduling (CALL timetable.f_control_importers()) made it reliably fatal.

Crash fix:
- Read the user mapping BEFORE creating the runtime, so a longjmp from
  GetUserMapping can no longer skip a live runtime.
- Use a current-thread tokio runtime (a Postgres backend is single
  threaded and the FDW only ever block_on's), removing the leaked
  worker-thread failure mode entirely.
- Connect to etcd lazily in begin_scan / the modify path instead of in
  new(), which the framework may call at planning time.

Robustness:
- iter_scan: convert non-UTF-8 etcd keys/values lossily instead of
  panicking, and iterate by index instead of draining the vec front
  (was O(n^2) per scan).
- begin_scan: do not panic when incrementing a prefix yields invalid
  UTF-8; fall back to the wider, still-correct range.
- insert/update: return an error on NULL key/value cells instead of
  panicking; do not panic on an empty user-mapping options list.
- update/delete: error on a missing key instead of silently creating it.

Tests:
- Add a regression test asserting that a scan with no user mapping
  returns a clean error instead of crashing the backend.

Build / reproducibility:
- Make testcontainers an optional dependency enabled by the `pg_test`
  feature (built for `cargo pgrx test`, not for release) and drop the
  unused serde dependency.
- Track Cargo.lock (pins the wrappers git dependency) for reproducible
  builds.
- pg_regress: CREATE EXTENSION etcd_fdw (was a stale template name).

Author: postgresql007

.gitignore

@@ -3,7 +3,6 @@
 /target
 *.iml
 **/*.rs.bk
-Cargo.lock
 pg_regress/results
 pg_regress/regression.diffs
 pg_regress/regression.out