Read RECORD with the default csv reader, not str.splitlines()

[dylanpulver]

construct_record_file writes RECORD with csv.writer, so a path containing a newline comes out correctly quoted. Both read sites — WheelFile.validate_record and WheelFile.get_contents — called str.splitlines() before handing the result to parse_record_file, so installer cannot read the RECORD it just wrote.

Measured on 59f0a65, using installer's own writer and then its own reader:

wrote  '"demo/we\nird.py",sha256=...,6'
read   'demo/weird.py'           (the newline is gone)

validate_record() on that wheel:
  In demo-1.0-py3-none-any.whl, demo/we
  ird.py is not mentioned in RECORD

splitlines() also splits on characters csv does not treat as row separators. I ran the writer and then the reader for all ten: \n and \r come back with the character silently deleted; \v, \f, \x1c, \x1d, \x1e, \x85, U+2028 and U+2029 raise expected 3 elements, got 1. All ten read back byte-identical through csv.reader over the same file.

The recording-installed-projects spec pins the dialect: RECORD "must be readable with the default reader of Python's csv module", line terminator "either \r\n or \n". Interposing splitlines() is a different, larger set of terminators.

Fix is to hand the RECORD text to parse_record_file as io.StringIO(text, newline="").

Ran pytest tests/: 152 passed, coverage report still 100%. Reverting only src/installer/sources.py to origin/main fails both new cases (150 passed, 2 failed). splitlines(keepends=True) is the obvious narrower fix and it does pass the newline case — csv reassembles the quoted field from the kept terminators — but it still fails the U+2028 case, which is why the test parametrises both. ruff check reports the same 3 pre-existing PLR1704 on an unmodified checkout and none in what I touched.

I did not change parse_record_file itself beyond one docstring line; it already accepts any iterable of strings.

AI assistance: this change was drafted with Claude Opus 5 (claude-opus-5). The outputs above were produced by running installer's own writer and reader locally against this branch.