TypeError in asyncpg.connect() for specific parameters when values are not str enough

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When calling `asyncpg.connect()` with values that are simple `str`, things work as I expect them to work. Not all of the arguments are treated the same, though. When values that go through the `WriteBuffer` onto the wire are `str`, but maybe not only a `str`, `WriteBuffer.write_str` raises a `TypeError`. My concrete use case where I ran into this is connection parameters that were read from a TOML file using `tomlkit`, which produces `tomlkit.items.String` values, that *are* instances of `str`, but are not accepted.

The following example script demonstrates the issue using `enum.StrEnum`, which causes the same unexpected error:

```python
import asyncio
import enum

import asyncpg

class Connect(enum.StrEnum):
HOST = 'localhost'
USER = 'postgresql'

async def connect_host():
return await asyncpg.connect(host=Connect.HOST)

async def connect_host_user():
return await asyncpg.connect(host=Connect.HOST, user=Connect.USER)

if __name__ == '__main__':
print('HOST is str:', isinstance(Connect.HOST, str))
print('USER is str:', isinstance(Connect.USER, str))

try:
asyncio.run(connect_host())
except asyncpg.PostgresError as e:
print('not connected:', repr(e))

try:
asyncio.run(connect_host_user())
except asyncpg.PostgresError as e:
# this except block is never hit, connection setup encounters a TypeError
print('not connected:', repr(e))
```

The output of this script is as follows (Python 3.14.6 on Linux amd64, asyncpg 0.31.0):

```traceback
HOST is str: True
USER is str: True
not connected: InvalidAuthorizationSpecificationError('role "user" does not exist')
Traceback (most recent call last):
File "asyncpg/protocol/protocol.pyx", line 978, in asyncpg.protocol.protocol.BaseProtocol.connection_made
File "asyncpg/protocol/coreproto.pyx", line 947, in asyncpg.protocol.protocol.CoreProtocol._connect
TypeError: Expected str, got Connect

During handling of the above exception, another exception occurred:

[...]

File "asyncpg/protocol/protocol.pyx", line 983, in asyncpg.protocol.protocol.BaseProtocol.connection_made
AttributeError: 'Protocol' object has no attribute '_on_error'
```

Using a enum here is a bit odd of course, the point is that the values being passed to `asyncpg.connect()` are `str` and treated differently depending on where that argument ends up in the connection setup.

My expectation is that values that are instances of `str` are used as such and work as intended. The full array connection parameters in my code are read through `tomlkit`, which produces values that very much quack and walk like a `str` 🦆.

Are these values being rejected for good reason, or should these just be accepted?

Inizia eseguendo il riproduttore dell’issue per enum.StrEnum, quindi esamina coreproto.pyx in corrispondenza di CoreProtocol._connect e il percorso WriteBuffer.write_str mostrato nel traceback. Traccia il motivo per cui i parametri di connessione vengono gestiti in modo diverso e aggiungi una copertura di regressione che dimostri che i valori compatibili con str segnalati vengono gestiti in modo coerente senza TypeError.

Scritto dal modello di indicizzazione a partire dal testo della issue.