libgstd: fix race conditions in pipeline list and session property access

[rwang-mtdata]

Summary

Thread-safety fixes for concurrent gstd clients accessing pipelines and session
properties. Addresses several races and a use-after-free observed under load.

Changes

• gstd_list: count pipelines under lock at point-of-use; lock shared linked access.
• gstd_list_reader / gstd_parser / gstd_pipeline: find_child now returns an
  owned reference, fixing a use-after-free when the child is removed concurrently.
• gstd_session: lock PROP_PIPELINES / PROP_DEBUG accessors; unref the old
  value when swapping.

Files changed

libgstd/gstd_list.c, gstd_list.h, gstd_list_reader.c, gstd_parser.c,
gstd_pipeline.c, gstd_session.c (+116 / -55)

Testing

• Built against current upstream/develop (rebased onto e95cd75).
• Internal CI (mtdata) build: passing.
• Running our system over 24 hours continously, then multiple restarts.

Risk

• N/a