CsvReadOptions.null_regex has no effect: matching values are read as literal strings, and fail the read in a numeric column

Start in crates/core/src/options.rs and datafusion-datasource-csv's src/source.rs:187, then compare the reader builder with the null_regex handling in src/file_format.rs:547. Add coverage to python/tests/test_context.py using a matching value in a numeric column, and run the CSV option tests. Done means matching fields become NULL for both string and numeric columns, while the documented behavior in docs/source/user-guide/io/csv.md remains accurate.

Describe the bug

CsvReadOptions.null_regex is accepted everywhere it is offered, but the CSV
reader never applies it. Values matching the regex are read as literal strings,
and when a matching value sits in a column typed as an integer the read fails
outright instead of producing NULL.

This is not a bug in this crate — python/datafusion/options.py stores the
value and crates/core/src/options.rs copies it into DataFusion's
CsvReadOptions correctly. The root cause is in DataFusion itself; see below.

To Reproduce

from pathlib import Path
from datafusion import CsvReadOptions, SessionContext

p = Path("probe.csv")
p.write_text("id,name\n1,alice\n2,N/A\n3,carol\n")

ctx = SessionContext()
options = CsvReadOptions().with_has_header(True).with_null_regex(r"^(null|NULL|N/A)$")
ctx.read_csv(p, options=options).show()

+----+-------+
| id | name  |
+----+-------+
| 1  | alice |
| 2  | N/A   |   <- expected NULL
| 3  | carol |
+----+-------+

Every entry point behaves the same way — the CsvReadOptions(null_regex=...)
constructor, with_null_regex(), read_csv(), register_csv(), and SQL:

ctx.sql("""
    CREATE EXTERNAL TABLE t (id INT, name VARCHAR)
    STORED AS CSV LOCATION 'probe.csv'
    OPTIONS ('format.has_header' 'true', 'format.null_regex' '^(null|NULL|N/A)$')
""")
ctx.sql("select * from t").show()   # same output, N/A not nulled

The SQL form does not reject the option, and giving an explicit schema does not
change anything, so this is not schema inference choosing Utf8.

When the matching value is in a numeric column, the read fails rather than
returning the wrong value:

p.write_text("id,value\n1,10\n2,N/A\n3,30\n")
ctx.sql("""CREATE EXTERNAL TABLE t2 (id INT, value BIGINT) STORED AS CSV
           LOCATION 'probe.csv'
           OPTIONS ('format.has_header' 'true', 'format.null_regex' '^(null|NULL|N/A)$')""")
ctx.sql("select * from t2").show()

DataFusion error: Arrow error: Parser error: Error while parsing value 'N/A' as
type 'Int64' for column 1 at line 2. Row data: '[2,N/A]'

This is the case that matters in practice: N/A, NULL and - placeholders
in otherwise numeric columns are the reason to reach for null_regex at all.

Expected behavior

A field matching null_regex is read as NULL, whatever the column's type.

Additional context

Why the existing coverage does not catch it: test_read_csv_with_options in
python/tests/test_context.py does set null_regex="[pP]+aris", but the only
Paris in its fixture is on the #Charlie;35;Paris line, which comment="#"
removes before the reader sees it. The None in that test's expected output
comes from truncated_rows=True on the Bob;25 row, not from null_regex. The
test pins that the option parses, which is what its comment says it is for — it
does not pin the behavior.

docs/source/user-guide/io/csv.md documents with_null_regex as "Treat these
as NULL", so the documented behavior and the actual behavior disagree.

Root cause (upstream). In datafusion-datasource-csv 55.0.0, the version
this repository pins, null_regex is applied during schema inference but never
to the reader that actually parses the rows:

• src/file_format.rs:547 sets it on the arrow::csv::reader::Format used for
  infer_schema.
• src/source.rs:187 builds the csv::ReaderBuilder that reads the data, and
  calls with_delimiter, with_header, with_quote, with_truncated_rows,
  with_terminator, with_escape and with_comment — but not
  with_null_regex. arrow_csv::reader::ReaderBuilder::with_null_regex exists
  in arrow 59.2.0 and is simply never called.

CsvSource already holds the full CsvOptions, so self.options.null_regex is
in scope at that point — it looks like a few lines in builder(), mirroring the
escape and comment blocks just below it.

I could not find this reported in either this repository or apache/datafusion.
If you would rather track it upstream, I am happy to open it against
apache/datafusion and link it back here.

Found while working on #1728 / #1732, which is why the advanced-options example
there keeps with_null_regex set but puts its N/A in a string column, so the
example runs. Happy to take the fix if it turns out to be on this side.