Add CursorQueryState result-set state machine

Model the lifecycle of a cursor's most recent query as an explicit state
machine (Idle / Active / Closed) so that illegal field combinations are
unrepresentable and fetching past the end of a result set is a clean,
specific error rather than the spurious "connection closed" you get from
re-polling a finished stream. The row stream is fused so an
exhausted-but-not-yet-reported stream can sit safely in `Active`.

This change adds `fetch_one`, `fetch_all` and `rowcount` (the batched
`fetch_next_batch` follows separately). On a stream error a cursor resets
to `Idle`; on normal exhaustion it moves to `Closed`, retaining the
PEP-249 rowcount from the command tag.

The state machine is generic over the stream type, defaulting to
`RowStream`, via a small `CursorRowStream` trait that abstracts the three
things the logic needs (the affected-row count, row->PyTuple conversion,
and error rendering). This is what lets the state transitions, exhaustion
handling and error recovery be unit-tested against an in-memory fake
stream, with no live Postgres server.

`cursor_state` is `pub` for now so its not-yet-consumed items don't trip
clippy's dead_code lint; the connection code wires it up and tightens that
later.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

rust/src/database/postgres/mod.rs

@@ -16,6 +16,7 @@ use pyo3::types::PyModule;
 // stops clippy's `dead_code` lint from firing on them before the
 // cursor/connection code (added in later changes) wires them up; the visibility
 // is tightened back to private once that happens.
+pub mod cursor_state;
 pub mod helpers;
 pub mod value;