Install pre-close vtab on readonly connections

[simolus3]

Some functions using StorageAdapter state may reasonably be called on read-only connections:

1. powersync_offline_sync_status() is read-only, and uses the adapter since Use microsecond precision for timestamps #183.
2. powersync_control('target_checkpoint_request_id', null) (added in 0.5.1) is read-only, and also uses the adapter.

Using the storage adapter is generally a good thing because it caches prepared statements. However, these statements need to be finalized before the database can fully close. For write connections, SDKs call powersync_init() which registers a custom vtab for this purpose. This function is not called on read-only connections though, so these statements leak and the connection is not properly closed.

As a simple fix that won't require SDK changes, this installs the pre-close hook only when it's needed to clean something up: When hooks are installed or when the storage adapter is created. For this to work on read-only connections, the vtab is now based on SELECT statements as INSERTs, even into virtual tables, are forbidden.

AI use: Migrating the vtab was done by Claude Code.

[rkistner]

The approach sounds good to me. Just want to check: How often would ensure_has_internal_close_vtab be called now? Just to confirm we're not effectively just preparing SELECT 1 FROM powersync_internal_close on every call now, which would kinda defeat the purpose of using prepared statements. It looks like it's called at most once per connection, but just want to confirm.

[simolus3]

Just to confirm we're not effectively just preparing SELECT 1 FROM powersync_internal_close on every call now, which would kinda defeat the purpose of using prepared statements

It's just done once per connection, I also confirmed this with breakpoints to be sure. The only call site that creates a new storage adapter is DatabaseState::storage_adapter, and the only way for for the adapter to be dropped is for release_resources to be called, which only happens when the connection is closed.