Fix #20: implement Connection.isValid() and track close state

[IceboxDev]

Fixes #20.

LibSqlConnection previously inherited the no-op isValid() from AbstractJdbcConnection, which throws SQLFeatureNotSupportedException. This breaks HikariCP / Spring Boot pool validation and causes JetBrains DataGrip / PyCharm to hang for ~50 s after every query while waiting for the unanswered validation call.

Three coupled changes, all in LibSqlConnection:

1. closed state field. isClosed() previously returned hardcoded false and close() didn't track state. A correct isValid() must short-circuit when the connection is closed, so the close flag has to come along.
2. isValid(int timeout) override. Runs SELECT 1 via the same LibSqlUtils.executeQuery path the constructor already uses to verify the connection. Wrapped in a CompletableFuture so the spec-mandated timeout actually applies (get(timeout, SECONDS) for timeout > 0, blocking get() for timeout == 0). Returns false (does not throw) on any HTTP / SQL failure, per the JDBC contract that isValid() is a predicate.
3. timeout < 0 ? SQLException per the JDBC spec.

Verified

mvn -f aggregate/pom.xml package and ... test both pass.

Manual integration run against Turso:

• isValid(5) ? true
• isValid(0) ? true
• isValid(-1) ? throws SQLException("Invalid timeout value: -1")
• isClosed() ? false before close, true after
• isValid(5) after close ? false

LibSqlDriverTest.main() was extended with two of those assertions.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 9 complexity · 0 duplication

Metric	Results
Complexity	9
Duplication	0

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[IceboxDev]

Related (independent) fixes opened in #52 (auth header on /version), #53 (fallback when /version is unavailable), #54 (getUpdateCount -1 for SELECT). All four together unblock JetBrains IDEs against Turso.