[Python] Don't skip a delimiter right after an escaped one in ReadFromText

[anishmehta24]

With escapechar set, ReadFromText skipped a delimiter that comes right after an escaped one, so b'a\\\n\nb\n' read as ['a\\\n\nb'] instead of ['a\\\n', 'b'] (same with custom delimiters).

_find_separator_bounds resumed the search one byte past the escaped delimiter; it now resumes right after it. Added a test that fails on master; the TextSourceTest tests in textio_test.py pass.

fixes #40459

Written with help from Claude.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.55%. Comparing base (69bdf5f) to head (1edd338).
⚠️ Report is 44 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40460       +/-   ##
=============================================
+ Coverage     59.04%   79.55%   +20.51%     
=============================================
  Files          2797      487     -2310     
  Lines        280750    82225   -198525     
  Branches      12488        0    -12488     
=============================================
- Hits         165764    65413   -100351     
+ Misses       108540    16812    -91728     
+ Partials       6446        0     -6446     

Flag	Coverage Δ
go	?
java	?
python	79.55% <100.00%> (-0.12%)	⬇️

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[damccorm]

Thanks, this LGTM