[RavSinghChandan]

Pulled on another one of the typeguard failures from #3689 and this one
turned out to be a real defect rather than test noise.

PdfWriterProtocol declares _reader, but PdfWriter only assigns it in the
incremental branch of init. So a writer built the normal way doesn't
actually satisfy the protocol it gets passed as — isinstance(writer,
PdfWriterProtocol) is False, and anything doing a runtime protocol check
rejects it. Under make testtype it's 62 of the failures, all on that one
missing attribute.

Declared it Optional[PdfReader] defaulting to None, next to the other
attribute docstrings in init. The three places that read it
(write(), _write_increment() twice) are all already inside
if self.incremental, so nothing changes at runtime — they just needed an
assert to narrow the Optional, which is the same assert ..., "mypy" style
already used elsewhere in this file.

Added a test that fails on main with AttributeError. mypy on pypdf/ reports
the same 22 pre-existing errors before and after, none in these files; the
offline suite goes 1007 → 1008 passed with the only failure being
test_appearance_stream_rtl, which fails the same way on a clean checkout here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.93%. Comparing base (1d92adf) to head (12d7589).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3960   +/-   ##
=======================================
  Coverage   97.93%   97.93%           
=======================================
  Files          57       57           
  Lines       10936    10940    +4     
  Branches     2048     2048           
=======================================
+ Hits        10710    10714    +4     
  Misses        125      125           
  Partials      101      101           

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[RavSinghChandan]

Both trimmed — dropped the backticks since these are plain comments rather
than rendered docstrings, and cut the second line. Left the one on
_write_increment, since "only reached from write() inside if self.incremental"
is the reason the assert is safe and isn't obvious from the call site.