`get_settings()` can get out of sync after a transaction with an error is rolled back

还没有人认领这个 Issue。

* **asyncpg version**: 0.30.0
* **PostgreSQL version**: 17.2
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: No
* **Python version**: 3.12.2
* **Platform**: Linux
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Yes
* **If you built asyncpg locally, which version of Cython did you use?**:
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: Only checked asyncio but it doesn't seem likely to matter

```
#!/usr/bin/env python3

import asyncio
import asyncpg

async def _check_encoding(con):
current_encoding = (await con.fetch(
"select current_setting('client_encoding')")
)[0]['current_setting']
print(current_encoding)
print(con.get_settings().client_encoding)

async def test(dsn):
con = await asyncpg.connect(dsn)

print("Orig")
await _check_encoding(con)

tran = con.transaction()
await tran.start()

await con.execute("set client_encoding to 'latin1'")
print("In transaction:")
await _check_encoding(con)
try:
await con.fetch("select 1 + 'a'")
except Exception:
pass

await tran.rollback()
print("After rollback:")
await _check_encoding(con)

res = await con.fetch('select $1::text', '💩')
print(res)

asyncio.run(test(
'postgres:///main?user=edgedb&port=5656&host=localhost'
))
```

In a transaction, if I set `client_encoding` to something, and then the transaction has an error and is rolled back, the `client_encoding` that `get_settings()` returns is never reverted.
If the transaction is rolled back without an error occuring, the right thing happens.

Output:
```
Orig
UTF8
UTF_8
In transaction:
LATIN1
LATIN1
After rollback:
UTF8
LATIN1
Traceback (most recent call last):
File "asyncpg/protocol/prepared_stmt.pyx", line 175, in asyncpg.protocol.protocol.PreparedStatementState._encode_bind_msg
File "asyncpg/protocol/codecs/base.pyx", line 227, in asyncpg.protocol.protocol.Codec.encode
File "asyncpg/protocol/codecs/base.pyx", line 129, in asyncpg.protocol.protocol.Codec.encode_scalar
File "asyncpg/pgproto/./codecs/text.pyx", line 29, in asyncpg.pgproto.pgproto.text_encode
File "asyncpg/pgproto/./codecs/text.pyx", line 17, in asyncpg.pgproto.pgproto.as_pg_string_and_size
UnicodeEncodeError: 'latin-1' codec can't encode character '\U0001f4a9' in position 0: ordinal not in range(256)
```

I would expect
```
After rollback:
UTF8
UTF8
```
and no encoding error.

从提供的 Python 复现程序开始，重点关注事务回滚路径和连接的 get_settings() 入口点。确认事务错误后的回滚与干净回滚之间的差异，然后追踪 client_encoding 状态是如何刷新的；当 get_settings() 在回滚后报告 UTF8，且后续的文本参数查询成功时，即表示完成。

由索引模型根据 Issue 内容生成。