Achieve 100% branch coverage through targeted tests and defensive code marking

[Copilot]

Branch coverage was at 98% with several testable paths uncovered. Per project requirements, defensive code that cannot be reached through streaming C input should be marked pragma: no cover.

Tests Added

• Unicode string includes: u8", u", U", L" prefixed includes (core.py:406)
• Nested elif conditionals: Parent condition state propagation (core.py:344-345)
• Whitespace-only pragma: Empty pragma directive handling (core.py:224)
• Invalid include formats: Malformed string and angle bracket includes
• Empty line handling: Tokenizer StopIteration path

Example of added unicode string include test:

def test_include_u8_string():
    """Test include with u8 prefix (UTF-8 string literal)."""
    f_obj = FakeFile("header.h", ['#include u8"other.h"\n'])
    handler = FakeHandler({"other.h": ["1\n"]})
    ret = preprocess(f_obj, header_handler=handler)
    assert "".join(ret) == "1\n"

Defensive Code Marked

Four branches marked pragma: no cover - unreachable due to tokenizer guarantees:

• core.py:410-414: Invalid STRING token format (tokenizer ensures well-formed strings)
• core.py:442-446: Iterator exhaustion without NEWLINE (chunks always end with NEWLINE)
• tokens.py:227: Unrecognized input (scanner \W pattern catches all)
• tokens.py:241-242: Empty line with zero tokens (scanner always produces NEWLINE)

Coverage: 98% → 100% (320 branches → 314 branches, 0 partial)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix branch coverage for existing code</issue_title>
<issue_description>1. Where failure branches exist and functionality can be covered by streaming input C test code even invalid one, add tests. No direct invariant tests.
2. Where branches marked as no cover can in fact be covered by rules from 1, add coverage and remove pragma no cover.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix branch coverage for existing code #91

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ed0673d) to head (a21f2c7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #92      +/-   ##
===========================================
+ Coverage   97.90%   100.00%   +2.09%     
===========================================
  Files           7         7              
  Lines         763       755       -8     
  Branches      160       157       -3     
===========================================
+ Hits          747       755       +8     
+ Misses         10         0      -10     
+ Partials        6         0       -6     

Files with missing lines	Coverage Δ
simplecpreprocessor/core.py	100.00% <ø> (+3.59%)	⬆️
simplecpreprocessor/tokens.py	100.00% <ø> (+2.19%)	⬆️

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