List[str] parameter treated as text instead of text[] in certain requests

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* **asyncpg version**: 0.27.0
* **PostgreSQL version**: 15.1
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: No
* **Python version**: 3.8.10
* **Platform**: Ubuntu
* **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)?**: Yes

I'm working with a custom bulk operations API built on top of SQLAlchemy and asyncpg, and some scenarios the List parameters are not treated as postgres arrays.

Example 1:
```
import asyncio
import asyncpg

async def try_it(table: str, query: str, *params, **connargs):
conn = await asyncpg.connect(**connargs)
try:
await conn.execute(f'CREATE TABLE test_table({table});')
await conn.execute(query, *params)
finally:
await conn.execute('DROP TABLE test_table;')
await conn.close()

table = 'a text[], b text'
query = """
UPDATE test_table SET a = uvals.a
FROM (VALUES ($1, $2)) AS uvals (a, b)
WHERE test_table.b = uvals.b
"""
params = [ ['hello', 'world'], 'helloworld']

db_conn_params = {}
asyncio.get_event_loop().run_until_complete(try_it(table, query, *params, **db_conn_params))
```

Response:
```
asyncpg.exceptions.DatatypeMismatchError: column "a" is of type text[] but expression is of type text
```

Example 2:
```
# same imports and try_it() from above

table = 'a text, b int'
query = """
SELECT a, b
FROM test_table
UNION
SELECT
values as a,
5 as b
FROM unnest($1) as values
"""
params = [ ['hello', 'world'] ]

# execution as above
```

Response:
```
asyncpg.exceptions.AmbiguousFunctionError: function unnest(unknown) is not unique
```

Adding a `$1 :: text[]` solves the problem in each case, since it appears to be passing the list as text but there are scenarios where I don't have direct control of the SQL (it being auto-generated).

Run the two try_it() reproductions against asyncpg 0.27.0 and PostgreSQL 15.1, comparing them with the explicit ::text[] casts. Trace how List parameters are inferred and encoded for VALUES and unnest queries. Done means both examples treat the list as text[] without requiring SQL casts, with regression coverage for the reported cases.

Written by the indexing model from the issue text.