[quinnj]

Closes #13. Detects idle-terminated sessions before pool reuse, without sending a probe query or retrying any user operation.

isopen reports local socket state. A server FATAL and EOF can already be waiting in the receive queue while that state still says open. Checking for one pending byte does not solve this safely: the rest of the PostgreSQL message or encrypted record may not have arrived.

Postgres.isvalid(conn) now collects available input through Reseau's non-blocking tryread! APIs. Validation and all normal post-authentication reads share one buffered stream, including startup session settings and reconnects. Only complete PostgreSQL messages are parsed. Partial messages remain available to a later validation, query, or notification read. GSS-encrypted frames also preserve partial headers and bodies.

• Fatal server messages, EOF, invalid framing, and transport failures make validation return false and close the transport.
• Notices, notifications, and parameter changes are handled normally. An in-use connection is left untouched. A true result means no known failure was observed; it is not proof of server liveness.
• Validation collects at most 128 KiB per call. Pool checkout takes exclusive ownership first, then validates outside the shared pool lock, so a waiting user callback cannot stall other checkouts. Exceptions return the pool permit.
• Postgres.isvalid is qualified public API, not exported and not an extension of Base.isvalid. isopen keeps its existing local-state meaning.
• The exact-read helper retained in this PR rejects truncated message bodies instead of parsing short input as valid data.

Validation:

• Full local Docker suite passed 6,404 checks on macOS / Julia 1.12.6 with two threads and coverage, including plain TCP, TLS, GSS, dead-peer replacement, session setup, transactions, COPY, and reconnects.
• Final focused tests passed another 151 assertions covering fragmented GSS input and EOF, partial PostgreSQL messages handed to normal reads, independent pool checkout during a waiting callback, and permit cleanup after a throwing callback.
• Local documentation build succeeded. All 16 CI checks pass at 3013885, including cross-platform tests, PostgreSQL-version tests, documentation, and coverage checks.

Depends on JuliaServices/Reseau.jl#163. Version remains 2.2.0. The four temporary CI installs of Reseau's nonblocking-peek branch must be removed after Reseau 1.5.0 is registered.

Co-authored by Codex