KeyError when clearing Connector._cache

### Bug Description

`KeyError` raised at this line:
https://github.com/GoogleCloudPlatform/cloud-sql-python-connector/blob/21a38d602beb980d80cf4f414773c1cd2b3bdd7d/google/cloud/sql/connector/connector.py#L421C9-L421C79

### Example code (or command)

```Python
import asyncio
import functools
from typing import Any

import asyncpg
import asyncpg.connection
import google.auth
import google.oauth2.service_account
from google.cloud.sql.connector import Connector, create_async_connector

from shared.settings import settings

_GCLOUD_CONNECTORS: dict[int, tuple[Connector, str | None]] = {}

async def _gcloud_async_creator() -> asyncpg.Connection[asyncpg.Record]:
return await _gcloud_connect(
dsn=settings.database.gcloud_dsn,
user=settings.database.user,
database=settings.database.name,
)

@functools.wraps(asyncpg.connection.connect)
async def _gcloud_connect(
dsn: str,
*,
database: str | None = None,
user: str | None = None,
**kwargs: Any,
) -> asyncpg.Connection[asyncpg.Record]:
loop = asyncio.get_running_loop()
loop_id = id(loop)
if loop_id not in _GCLOUD_CONNECTORS:
credentials, _ = google.auth.default()
user_id = (
credentials.service_account_email
if isinstance(credentials, google.oauth2.service_account.Credentials)
else None
)
_GCLOUD_CONNECTORS[loop_id] = (
await create_async_connector(credentials=credentials),
gcloud_db_user(user_id) if user_id else None,
)

connector, credential_user = _GCLOUD_CONNECTORS[loop_id]

if credential_user is not None:
user = credential_user

return await connector.connect_async(
dsn,
"asyncpg",
db=database,
user=user,
enable_iam_auth=True,
**kwargs,
)

def gcloud_db_user(user_id: str) -> str:
"""
Return the Google Cloud DB user name for the given Google Cloud user.
"""
return user_id.removesuffix(".gserviceaccount.com")
```

### Stacktrace

```bash
KeyError: ('xyz:europe-west3:main', True)
...
File "sqlalchemy/ext/asyncio/session.py", line 463, in execute
result = await greenlet_spawn(
File "sqlalchemy/util/_concurrency_py3k.py", line 201, in greenlet_spawn
result = context.throw(*sys.exc_info())
File "kron/db.py", line 193, in execute
result = super().execute(statement, *args, **kwargs)
File "sqlalchemy/orm/session.py", line 2365, in execute
return self._execute_internal(
File "sqlalchemy/orm/session.py", line 2241, in _execute_internal
conn = self._connection_for_bind(bind)
File "sqlalchemy/orm/session.py", line 2110, in _connection_for_bind
return trans._connection_for_bind(engine, execution_options)
File "", line 2, in _connection_for_bind
File "sqlalchemy/orm/state_changes.py", line 139, in _go
ret_value = fn(self, *arg, **kw)
File "sqlalchemy/orm/session.py", line 1189, in _connection_for_bind
conn = bind.connect()
File "sqlalchemy/engine/base.py", line 3273, in connect
return self._connection_cls(self)
File "sqlalchemy/engine/base.py", line 145, in __init__
self._dbapi_connection = engine.raw_connection()
File "sqlalchemy/engine/base.py", line 3297, in raw_connection
return self.pool.connect()
File "sqlalchemy/pool/base.py", line 449, in connect
return _ConnectionFairy._checkout(self)
File "sqlalchemy/pool/base.py", line 1264, in _checkout
fairy = _ConnectionRecord.checkout(pool)
File "sqlalchemy/pool/base.py", line 713, in checkout
rec = pool._do_get()
File "sqlalchemy/pool/impl.py", line 179, in _do_get
with util.safe_reraise():
File "sqlalchemy/util/langhelpers.py", line 224, in __exit__
raise exc_value.with_traceback(exc_tb)
File "sqlalchemy/pool/impl.py", line 177, in _do_get
return self._create_connection()
File "sqlalchemy/pool/base.py", line 390, in _create_connection
return _ConnectionRecord(self)
File "sqlalchemy/pool/base.py", line 675, in __init__
self.__connect()
File "sqlalchemy/pool/base.py", line 901, in __connect
with util.safe_reraise():
File "sqlalchemy/util/langhelpers.py", line 224, in __exit__
raise exc_value.with_traceback(exc_tb)
File "sqlalchemy/pool/base.py", line 897, in __connect
self.dbapi_connection = connection = pool._invoke_creator(self)
File "sqlalchemy/pool/base.py", line 362, in
return lambda rec: creator_fn()
File "sqlalchemy/ext/asyncio/engine.py", line 115, in creator
return sync_engine.dialect.dbapi.connect( # type: ignore
File "sqlalchemy/dialects/postgresql/asyncpg.py", line 961, in connect
await_only(creator_fn(*arg, **kw)),
File "sqlalchemy/util/_concurrency_py3k.py", line 132, in await_only
return current.parent.switch(awaitable) # type: ignore[no-any-return,attr-defined] # noqa: E501
File "sqlalchemy/util/_concurrency_py3k.py", line 196, in greenlet_spawn
value = await result
File "kron/db.py", line 435, in _gcloud_async_creator
return await _gcloud_connect(
File "kron/db.py", line 423, in _gcloud_connect
return await connector.connect_async(
File "/srv/venv/apiserver/kron-9TtSrW0h-py3.13/lib/python3.13/site-packages/google/cloud/sql/connector/connector.py", line 366, in connect_async
await self._remove_cached(str(conn_name), enable_iam_auth)
File "/srv/venv/apiserver/kron-9TtSrW0h-py3.13/lib/python3.13/site-packages/google/cloud/sql/connector/connector.py", line 421, in _remove_cached
cache = self._cache.pop((instance_connection_string, enable_iam_auth))
```

### Steps to reproduce?

I've observed just one occurrence of the issue so far and it seemed pretty random. Unfortunately I'm not able to reproduce.

### Environment

1. OS type and version: Debian GNU/Linux 12 (bookworm)
2. Python version: 3.13.4
3. Cloud SQL Python Connector version: 1.18.2

### Additional Details

_No response_