fix: apply CSV null_regex when reading, not only when inferring the schema

[Developer1010x]

Which issue does this PR close?

• Closes CSV null_regex is applied to schema inference but never to the reader, so matching values are not null #25213.

Rationale for this change

Setting null_regex on a CSV source has no effect on the data that comes back.
A field matching the pattern is read as the literal string, and if it lands in a
column typed as a number the query fails outright:

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

That second case is the one that matters in practice — N/A, NULL and -
placeholders are almost always sitting in columns that are otherwise numeric,
which is the reason to reach for null_regex in the first place.

The option was applied to the arrow::csv::reader::Format used by
CsvFormat::infer_schema, but never to the ReaderBuilder that parses the
rows. Inference therefore behaves as though the regex were honored and only the
data disagrees, which is what makes it easy to miss.

What changes are included in this PR?

• CsvSource::null_regex(), alongside the existing escape(), comment() and
  terminator() accessors.
• CsvSource::builder() applies the regex, and now returns Result so an
  invalid pattern surfaces as a configuration error instead of panicking.
  CsvFormat::infer_schema currently .expect()s on the same regex, so a
  malformed pattern is a panic there today; this side no longer adds a second
  one.
• Three call sites updated for the new signature: CsvSource::open, and the two
  streaming decoder paths in CsvOpener::open (the byte-range branch and the
  GetResultPayload::Stream branch). Those two go through builder() as well,
  so they were missing the regex for the same reason.

The fix is four lines; the signature change is what makes it touch more.

What is the testing strategy for this PR?

There was no coverage of null_regex anywhere in the repository before this
change — not in datafusion/datasource-csv, not in the sqllogictest files.

Unit tests in datafusion/datasource-csv/src/source.rs:

• null_regex_nulls_matching_string_values
• null_regex_nulls_matching_values_in_numeric_columns — the case that errors
  today rather than returning a wrong value
• without_null_regex_the_placeholder_is_read_verbatim — control, pins that the
  unset behavior does not change
• invalid_null_regex_is_reported_as_an_error

I checked these are not vacuous: with the four added lines in builder()
removed, the three behavioral tests fail and the control still passes.

SQL-level coverage in datafusion/sqllogictest/test_files/csv_files.slt, over a
new datafusion/core/tests/data/null_regex.csv fixture, covering a matching
value in both a VARCHAR and an INT column.

Are there any user-facing changes?

null_regex starts doing what it is documented to do. Anything that set the
option and worked around it being ignored — for example by typing a column as
VARCHAR to avoid the parse error, then filtering the placeholder out in SQL —
will now see NULL where it previously saw the literal string.

No public API changes: builder() is private, and the new null_regex()
accessor is additive.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.73913% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.88%. Comparing base (9082d6b) to head (7ac3811).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/datasource-csv/src/source.rs	96.73%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25254   +/-   ##
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  Coverage   81.88%   81.88%           
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  Files        1133     1133           
  Lines      424522   424609   +87     
  Branches   424522   424609   +87     
=======================================
+ Hits       347623   347701   +78     
- Misses      56285    56290    +5     
- Partials    20614    20618    +4     

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