fix(csv): strip leading byte-order mark in CsvParseStream

[LeSingh1]

The synchronous parse() function already strips a leading UTF-8
byte-order mark (U+FEFF) from its input, but CsvParseStream did not.

When a CSV file begins with a BOM -- common output from Excel and other
Windows tools -- the first field name arrives as "name" instead
of "name". That corrupts header-based lookups silently:

const source = ReadableStream.from(["name,age\n", "Alice,34\n"]);
const records = await Array.fromAsync(
  source.pipeThrough(new CsvParseStream({ skipFirstRow: true })),
);
// before this fix:
// [{ "name": "Alice", age: "34" }]   -- BOM leaks into key
// after:
// [{ name: "Alice", age: "34" }]

The fix adds a #firstLine flag to StreamLineReader and strips the
BOM from the first line it reads, exactly matching what parse() does
via its BYTE_ORDER_MARK constant.

Two new tests cover the regression: one for plain string[][] output and
one for skipFirstRow: true (object output, where the BOM corrupts the
header key).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (9e34775).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7183      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      637       +1     
  Lines       52142    52159      +17     
  Branches     9401     9403       +2     
==========================================
+ Hits        49315    49328      +13     
- Misses       2249     2254       +5     
+ Partials      578      577       -1     

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