BREAKING(csv): honour fieldsPerRecord when mapping rows to objects

[fibibot]

Summary

Fixes #6434.

CsvParseStream and parse() document fieldsPerRecord: -1 (or undefined,
the default) as allowing variable-length records, and that worked when
emitting string[][]. Once skipFirstRow: true or columns: [...] was set,
parsing threw Syntax error on line N: The record has X fields, but the header has Y fields even with fieldsPerRecord: -1, because
convertRowToObject threw unconditionally on length mismatch, ignoring the
fieldsPerRecord setting.

This change makes convertRowToObject accept an allowVariableLength flag
that both call sites compute from the active fieldsPerRecord mode (the
stream's #fieldsPerRecord === "ANY"; for the sync parse(),
options.fieldsPerRecord === undefined || options.fieldsPerRecord < 0).

Runtime behaviour:

• Variable-length mode (fieldsPerRecord undefined or negative): short rows
  yield undefined for missing header keys, extra fields are dropped.
• Strict modes (fieldsPerRecord >= 0): unchanged. The existing
  expected N fields but got M check still fires before
  convertRowToObject, and the residual length-vs-header check still throws
  when columns differs in length from a positive fieldsPerRecord.

Type widening (BREAKING)

The mapped-row value type widens from Record<string, string> to
Record<string, string | undefined> (and the same shift inside
RecordWithColumn) so callers see the runtime possibility of undefined.
This is a TS-level breaking change for callers reading values cookie-style
without noUncheckedIndexedAccess. The issue calls this out; widening
unconditionally was preferred over gating on fieldsPerRecord because the
default mode (no fieldsPerRecord set) is variable-length, so any caller
using skipFirstRow or columns with the default fieldsPerRecord can now
observe undefined and the type should reflect that.

Test plan

• deno fmt --check csv/
• deno lint csv/
• deno test -A --doc csv/ (72 tests / 199 steps, all passing)
• Regression tests added for all three reproductions from CsvParseStream: negative fieldsPerRecord doesn't work with skipFirstRow or columns #6434
  (skipFirstRow / columns / both) in both csv/parse_test.ts and
  csv/parse_stream_test.ts.
• Existing strict-mode fieldsPerRecord error paths still tested.

Closes bartlomieju/orchid-inbox#88

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (5ea9159) to head (1b7ffac).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7141   +/-   ##
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  Coverage   94.61%   94.61%           
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  Files         634      634           
  Lines       51830    51838    +8     
  Branches     9341     9342    +1     
=======================================
+ Hits        49037    49045    +8     
  Misses       2218     2218           
  Partials      575      575           

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