BigQuery TIMESTAMP_MILLIS transform conflict when model substitutes

[goldmedal]

Describe the bug
When the model substitutes, we use sqlglot to transpile the dialect SQL to our dialect

wren-engine/ibis-server/app/mdl/substitute.py

Line 26 in 9c31174

ast = parse_one(sql, dialect=self.data_source.value)

However, after transforming to the sqlgot IR, the function name could be mapped to its function name.
In this case, TIEMSTAMP_MILLIS will be transformed to UNIX_TO_TIME when the output dialect is None (used by our v3 API). If the output dialect is trino (used by our v2 API), it will be transformed to FROM_UNIXTIME(CAST(timestamp AS DOUBLE) / POW(10, 3)). After the v2 wren core planning, the function name will be transformed to TIEMSTAMP_SECONDS(CAST(timestamp AS DOUBLE) / POW(10, 3)), which function signature is wrong. TIMESTAMP_SECONDS only accepts INT64, but the result SQL tries to input a divided number (float64).

To Reproduce
Steps to reproduce the behavior:
For v2 case:

import sqlglot

sql = """
SELECT timestamp_millis(timestamp) from ask_question_traces
"""
ast = sqlglot.parse_one(sql, read="bigquery")
bq = ast.sql(dialect="trino")
result = sqlglot.transpile(bq, read="trino", write="bigquery")[0]
print(result)

The result is

SELECT TIMESTAMP_SECONDS(CAST(timestamp AS FLOAT64) / POWER(10, 3)) FROM ask_question_traces

For v3 case:

import sqlglot

sql = """
SELECT timestamp_millis(timestamp) from ask_question_traces
"""
ast = sqlglot.parse_one(sql, read="bigquery")
ir_sql = ast.sql(dialect=None)
print(ir_sql)
result = sqlglot.transpile(ir_sql, read=None, write="bigquery")[0]
print(result)

The result is

SELECT UNIX_TO_TIME(timestamp, 3) FROM ask_question_traces
SELECT TIMESTAMP_MILLIS(timestamp) FROM ask_question_traces

The v3 one is better. There is a only problem for UNIX_TO_TIME isn't included in our function lists.