add cast, fix order by with expressions

[shcheklein]

Public cast API and expression order_by fixes

Summary

• added public func.cast(...)
• made cast usable in mutate(), filter(), order_by(), and merge()
• fixed order_by() expression handling to match partition_by more closely
• fixed SQLite datetime cast runtime behavior
• added docs and broader tests

Examples

Cast in regular expressions

dc.read_values(id_str=["10", "2", "1"]).mutate(
    id_int=func.cast("id_str", int)
).order_by(func.cast("id_str", int))

Merge on mismatched key types

left.merge(
    right,
    on=func.cast(left.c("id_str"), int),
    right_on=right.c("id_int"),
    inner=True,
)

order_by with expressions

dc.read_values(id=[2, 1, 3]).order_by(dc.C("id"), descending=True)

dc.read_values(a=[10, 1, 5], b=[1, 20, 2]).order_by(
    dc.C("a") + dc.C("b"),
    descending=True,
)

dc.read_values(file=[...]).order_by(
    func.string.length(dc.C("file.path")) + dc.C("file.size")
)

Datetime cast on SQLite

from datetime import datetime

dc.read_values(ts_str=["2024-01-02 03:04:05"]).mutate(
    ts=func.cast("ts_str", datetime)
)

Fixes

• resolve_columns() now handles string names, DataChain function expressions, and existing SQL expressions correctly for order_by()
• SQLite datetime casts now compile through datetime(...) instead of raw CAST(... AS DATETIME)
• DateTime read conversion now parses ISO-like strings back into Python datetime

Tests

• functional tests for cast in mutate(), filter(), order_by(), and merge()
• scalar runtime cast coverage for int, float, str, bool, datetime
• unit result-type coverage for int, float, str, bool, bytes, datetime, list[int], dict[str, int]
• expression order_by() coverage next to the existing order-by unit tests

Docs

• documented func.cast in the function reference

[codecov]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/datachain/sql/types.py	66.66%	2 Missing ⚠️

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