fix(jdbc): drop duplicate checkClosed in prepared execute (#150)

Fixes #81

Contract: Hrana 3 is the source of truth for transport, values,
metadata, batching, transactions, and error reporting; `java.sql` is the
source of truth for the JDBC surface; xerial/sqlite-jdbc is a behavioral
reference and conformance foundation, never a copy target.

## What changed

`LibsqlPreparedStatement.executeCurrent()` called `checkClosed();
beginExecution(); checkClosed();`. The second check runs after the state
transition and before `connection.executeStatement`, which already
checks the connection itself. Removed it, leaving the single
`checkClosed()` before the state transition.

One file changed, one line deleted, no behavior change.

## Divergence from the plan's Files block

`LibsqlStatement.java` is in the plan's Files block but needed no edit:
its second `checkClosed()` was already removed in `4b30b46` (#85), which
matched xerial's tolerance of `execute` after `statement.close()`. That
behavior is pinned by
`LibsqlStatementLifecycleTest.executeAndGetConnectionStillWorkAfterClose`
(#148), so adding a `checkClosed()` to the statement execute path would
be a behavior change and was not done. `executeSql` reads
`beginExecution();` with the connection check inside
`LibsqlConnection.executeStatement`.

## Evidence

Red at trunk (`origin/main` `de31dc6`): `executeCurrent` at lines
231-235 reads `checkClosed(); beginExecution(); checkClosed();`. Green
at head (`2621b71`): one check. Log:
`/tmp/xerial-parity-i81/lane-3-specific.log`.

Lanes at head (logs under `/tmp/xerial-parity-i81/`):

1. Regression lane: not applicable. The plan's unit box says no new
tests, so there is no new failing case; the diff check stands in for it
(lane 3).
2. Lifecycle and prepared suites, trunk vs head: identical, 21
`LibsqlStatementLifecycleTest` + 6 `LibsqlPreparedStatementTest` = 27/27
pass on both sides (`trunk-unit.log`, `head-unit.log`).
3. No double check remains in either file (`lane-3-specific.log`).
4. `-pl libsql-jdbc test`: BUILD SUCCESS, 293/293
(`lane-4-module-suite.log`); trunk is also 293/293
(`trunk-module-suite.log`).
5. `-pl libsql-conformance -am verify`: BUILD SUCCESS, 123 tests, 0
failures, 1 skipped, no divergence line (`lane-5-conformance.log`).
6. `-Pxerial-control verify`: BUILD SUCCESS, 100 tests, 0 failures, 1
skipped; differences are the pre-existing reference notes, none
introduced here (`lane-6-control.log`).
7. `spotless:check`: BUILD SUCCESS (`lane-7-spotless.log`).
8. `./mvnw test`: BUILD SUCCESS, 293/293 in `libsql-jdbc`
(`lane-8-reactor.log`).
9. Statement surface: BUILD SUCCESS, 30/30 (`lane-9-specific.log`).
10. `-pl libsql-jdbc -am package -DskipTests`: BUILD SUCCESS, driver jar
built (`lane-10-package.log`).

Perf: three interleaved `./mvnw -pl libsql-jdbc test -q` runs per side,
trunk first. Trunk median 2.161 s, head median 2.167 s (+0.3%, within
the 10% rule). Raw values in `perf-raw.txt`.

Base `de31dc6`, head `2621b71e5c77c5a9976519942e2b8d5b2044ff29`.

No merge; stopping at merge-ready.

Author: magnusp

libsql-jdbc/src/main/java/com/github/magnusp/libsql/jdbc/LibsqlPreparedStatement.java

@@ -231,7 +231,6 @@ public boolean execute() throws SQLException {
     private StmtResult executeCurrent() throws SQLException {
         checkClosed();
         beginExecution();
-        checkClosed();
         return connection.executeStatement(buildCurrentStmt());
     }