SRombauts / SQLiteCpp / 28507929241

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Fix Savepoint destructor exception-safety and rollback bookkeeping (#559)

## Summary

Fixes two findings from the deep code review (SP-03, SP-02) in the
`Savepoint` destructor. The documented auto-rollback-on-scope-exit
semantics are unchanged.

### SP-03 — destructor could call `std::terminate`
`~Savepoint()` caught only `SQLite::Exception`. Building the `"ROLLBACK
TO SAVEPOINT " + msName` / `"RELEASE SAVEPOINT " + msName` strings can
throw `std::bad_alloc`, and any non-`SQLite::Exception` thrown during
cleanup would escape the implicitly `noexcept` destructor and terminate
the process. The handler is now `catch (...)`, matching the stated
intent ("Never throw an exception in a destructor").

### SP-02 — no rolled-back state, fragile double-rollback
There was a single `mbReleased` flag and no notion of "already rolled
back", so after a manual `rollbackTo()` (or on the normal scope-exit
path) the destructor issued a `ROLLBACK TO` and then a `RELEASE`,
relying on SQLite tolerating a repeated `ROLLBACK TO`. A new
`mbRolledBack` flag (set in `rollbackTo()`) lets the destructor do the
minimum: skip the redundant `ROLLBACK TO` and just `RELEASE`.

## Test plan
- Added `Savepoint.rollbackToThenRelease` covering a manual
`rollbackTo()` followed by `release()` and a clean scope exit.
- Built `SQLiteCpp_tests` (MSVC, Debug); `ctest -C Debug` 100% passed,
including the existing `Savepoint.commitRollback` and
`Savepoint.destructorSwallowsException` tests.

No public API or behavioural change on the happy path; only the
(previously UB-on-OOM) destructor error handling and the
redundant-command path change.

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