Detect peer EOF and TLS closure before reusing idle connections. Make
disconnect idempotent while preserving buffered data and pending operation
lifetimes.
Keep the release branch's multi-byte recv_n behavior, and cover partial EOF,
socket modes, and TLS shutdown with loopback regression tests. Use fixed
test certificates and retain compatibility with the release branch's
Google Test version.
Co-authored-by: woodser <13068859+woodser@users.noreply.github.com>
(cherry picked from commit 096abdaa22b71ade9ca673ed8012c500cecb58d3)
