Support for bun (uptrace/bun) PostgreSQL driver

## Feature Request

Add support for [bun](https://github.com/uptrace/bun)'s PostgreSQL driver (`uptrace/bun/driver/pgdriver`) alongside the existing pgx and mysql drivers.

bun is a widely-used Go ORM/query builder (4.7k GitHub stars) that uses its own `database/sql`-compatible PostgreSQL wire protocol implementation (not pgx). Paxos uses it in production for Aurora PostgreSQL workloads that would benefit from the wrapper's failover, EFM, and IAM plugins.

## Current Blocker

The wrapper can't be used with bun today because `DialectManager.GetDialect()` hardcodes a check for the `awssql-pgx` driver name for all PostgreSQL connections:

```go
// dialect_manager.go
if strings.Contains(driverProtocol, "postgres") {
driverIsRegistered := d.FindRegisteredDriver(AWS_PGX_DRIVER_CODE)
if !driverIsRegistered {
return nil, error_util.NewGenericAwsWrapperError(...)
}
```

A custom `DriverDialect` implementation for bun registers under its own driver name (e.g. `awssql-bunpg`), but the dialect manager rejects it because `awssql-pgx` isn't registered. This blocks bun users from enabling failover, EFM, or any plugin that goes through dialect detection — even though the `DriverDialect` interface is public and fully implemented.

The same issue would affect anyone implementing a custom `DriverDialect` for another PostgreSQL driver (e.g., lib/pq).

## Proposal

Two approaches (not mutually exclusive):

### 1. Remove the hardcoded driver registration check

The `FindRegisteredDriver(AWS_PGX_DRIVER_CODE)` check in `GetDialect()` is redundant — if no driver is registered, the connection fails with a clear error at `sql.Open` / `Ping` time. Removing it unblocks custom `DriverDialect` implementations immediately. This is a small change (~20 lines removed).

We have a PR ready for this if you'd like to review it.

### 2. Add an official `bun-driver` module

Similar to the existing `pgx-driver/` and `mysql-driver/` modules, add a `bun-driver/` module that provides:

- `BunPgDriverDialect` — implements `DriverDialect` for bun's pgdriver (URL-format DSN, bun-specific error types)
- `BunPgErrorHandler` — error classification using `pgdriver.Error` (same SQLSTATE codes as pgx)
- `BunPgRowParser` — type assertions for bun's `driver.Value` types (identical to pgx — same wire protocol)
- `BunPgPropertyResolver` — DSN parameter mapping (identical to pgx — same PostgreSQL parameters)
- Driver registration via `init()` as `awssql-bunpg`

We've prototyped this at Paxos (a few hundred lines, mostly parallel to the pgx-driver module) and can contribute it as a PR. The main bun-specific difference is DSN format: bun requires URL-format (`postgres://user@host:port/db?params`) while pgx uses key=value format.

## Workaround

Our current workaround is registering a dummy driver under the `awssql-pgx` name to satisfy the check, which works but is hacky.

Start with dialect_manager.go and GetDialect(), then compare the existing pgx-driver/ and mysql-driver/ modules with the public DriverDialect interface. Clarify whether the driver-registration change, an official bun-driver/ module, or both are in scope; done means bun's PostgreSQL driver works with dialect detection and the wrapper plugins.

Written by the indexing model from the issue text.