ConnectionError from _cancel() during CancelledError not caught, crashes callers

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## Environment

- **asyncpg version**: 0.31.0 (also reproduced on 0.30.x)
- **PostgreSQL version**: 16
- **Python version**: 3.11.14
- **Platform**: Linux (Kubernetes)
- **pgbouncer**: No
- **SQLAlchemy**: 2.0.23

## Summary

When an asyncpg operation is cancelled via `asyncio.CancelledError` while mid-query, the cancellation mechanism in `connect_utils._cancel` can raise a **built-in `ConnectionError`** that escapes to the caller. This is problematic because:

1. Callers (e.g. SQLAlchemy) expect asyncpg-specific exception types and don't handle built-in `ConnectionError`
2. The cancel operation is inherently best-effort — if the cancel connection fails, the error should be suppressed or wrapped, not propagated

This is related to #1211 but occurs on **non-`direct_tls`** connections via the cancel request code path.

## Reproduction flow

1. An asyncpg connection is executing a query (e.g. inside SQLAlchemy's `session.execute()`)
2. The asyncio task is cancelled (`task.cancel()`)
3. `CancelledError` propagates into `protocol.query()` / `bind_execute`
4. asyncpg's cancellation handler tries to send a PostgreSQL cancel request by opening a **new** SSL connection via `connect_utils._cancel` → `_create_ssl_connection`
5. The new connection fails (server already closed the original, or network issue)
6. `TLSUpgradeProto.connection_lost()` raises built-in `ConnectionError('unexpected connection_lost() call')`
7. This escapes through `connect_utils._cancel` (which has **no error handling** around `_create_ssl_connection`)
8. Caller receives `ConnectionError` instead of `CancelledError`

## Traceback

```
asyncio.exceptions.CancelledError (original exception)

During handling of the above exception, another exception occurred:

File "asyncpg/transaction.py", line 206, in __rollback
await self._connection.execute(query)
File "asyncpg/connection.py", line 350, in execute
result = await self._protocol.query(query, timeout)
File "asyncpg/connection.py", line 1584, in _cancel
await connect_utils._cancel(
File "asyncpg/connect_utils.py", line 1040, in _cancel
tr, pr = await _create_ssl_connection(
File "asyncpg/connect_utils.py", line 752, in _create_ssl_connection
do_ssl_upgrade = await pr.on_data
^^^^^^^^^^^^^^^^
ConnectionError: unexpected connection_lost() call
```

## Root cause

Two issues in `connect_utils.py`:

### 1. `_cancel()` has no error handling around `_create_ssl_connection`

```python
async def _cancel(*, loop, addr, params, backend_pid, backend_secret):
...
if params.ssl and params.sslmode != SSLMode.allow:
tr, pr = await _create_ssl_connection(...) # ← no try/except!
...
```

The cancel request is best-effort (we're telling PostgreSQL to cancel a query on a connection that may already be dead). If opening the cancel connection fails, the error should be suppressed or wrapped in `asyncpg.InterfaceError`, not propagated as a raw `ConnectionError`.

### 2. `TLSUpgradeProto.connection_lost()` raises built-in `ConnectionError`

```python
def connection_lost(self, exc):
if not self.on_data.done():
if exc is None:
exc = ConnectionError('unexpected connection_lost() call')
self.on_data.set_exception(exc)
```

This raises a **built-in Python `ConnectionError`**, not an asyncpg exception type. Callers like SQLAlchemy check for `asyncpg.InterfaceError` or `asyncpg.PostgresError` to detect disconnects. A built-in `ConnectionError` bypasses all those checks, which means:
- SQLAlchemy's `is_disconnect()` doesn't recognize it
- SQLAlchemy's pool pre-ping handler (`_do_ping_w_event`) only catches `self.loaded_dbapi.Error`, so `ConnectionError` escapes
- The pool's retry logic (which would create a fresh connection) never triggers

## Suggested fix

**Option A** (minimal): Catch `OSError` (parent of `ConnectionError`) in `connect_utils._cancel()` and suppress it — cancel is best-effort:

```python
async def _cancel(*, loop, addr, params, backend_pid, backend_secret):
...
try:
if params.ssl and params.sslmode != SSLMode.allow:
tr, pr = await _create_ssl_connection(...)
...
except OSError:
# Cancel is best-effort. If we can't reach the server, the
# connection is dead anyway.
return
```

**Option B** (comprehensive): Also change `TLSUpgradeProto.connection_lost()` to raise `asyncpg.InterfaceError` instead of built-in `ConnectionError`, so callers can handle it consistently:

```python
def connection_lost(self, exc):
if not self.on_data.done():
if exc is None:
exc = InterfaceError('unexpected connection_lost() call')
self.on_data.set_exception(exc)
```

## Impact

This causes process crashes in production services. When a task is cancelled during a DB query, the `ConnectionError` escapes all exception handlers (which expect either `CancelledError` or asyncpg-specific exceptions) and terminates the process.

This is 100% correlated with `CancelledError` in our logs — every `ConnectionError: unexpected connection_lost()` we've seen is triggered by task cancellation.

## Additional context

We use Google CloudSQL with SSL connections. The PostgreSQL server is accessed over SSL (non-`direct_tls`), which means the cancel code path goes through `_create_ssl_connection` to establish a new SSL connection for sending the cancel request.

Comienza en connect_utils.py, en _cancel y _create_ssl_connection, y luego inspecciona TLSUpgradeProto.connection_lost() para entender cómo se traducen los fallos de cancelación. Añade una prueba de regresión para un fallo de cancelación de conexión SSL que impida que un ConnectionError sin procesar llegue al llamador y, después, ejecuta las pruebas relevantes de cancelación y conexión.

Escrito por el modelo de indexación a partir del texto del issue.