fix: reject multiple statements in single-statement APIs

[zfaustk]

Summary

Fixes #2244.

Single-statement Connection::execute and Connection::query calls now reject
multiple executable SQL statements before any statement is stepped. The
explicit execute_batch API keeps its existing multi-statement behavior.

The guard is applied to local connections and embedded-replica
RemoteConnection/RemoteStatement paths. This prevents the first statement
from partially mutating the database when a second statement is supplied.

Verification

• cargo test -p libsql --test integration_tests --no-default-features --features core,stream -- --nocapture (30 passed, 1 ignored)
• cargo test -p libsql --test replication --no-default-features --features replication -- --nocapture (2 passed)
• cargo check -p libsql --no-default-features --features core,replication (passed)
• cargo check -p libsql --no-default-features --features core,sync (passed)
• cargo check -p libsql --no-default-features --features core,remote (passed)
• cargo fmt --all -- --check (passed)
• git diff --check (passed)

The full all-feature workspace check was not completed locally because the
environment does not provide cmake; hosted CI will provide the remaining
matrix.

AI assistance was used for implementation and review; the final diff and
verification results were checked against the repository source and tests.