test: add Expat.pm input validation and state guard tests

[toddr-bot]

What

Add 22 tests covering previously untested defensive code paths in Expat.pm.

Why

Several core validation and state management paths had zero test coverage:

• setHandlers() input validation (non-CODE ref rejection, odd argument count, unknown types)
• Parse state guards (_State_ checks preventing concurrent/reuse scenarios)
• Reference exception preservation through the parse error handling path
• State-dependent method behavior outside active parsing

How

New test file t/expat_guards.t exercises each path directly:

• Tests 1-9: setHandlers validation — rejects strings, hashrefs, arrayrefs, numbers, scalar refs; accepts undef/0; detects odd args and unknown types
• Tests 10-12: Parse state guards — concurrent parse, reuse via parse(), reuse via parsefile()
• Tests 13-16: Reference exceptions — blessed object dies propagate correctly, with and without ErrorContext
• Tests 17-22: State methods return false outside parse context

Testing

Full test suite passes: 700 tests across 60 files.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.73%. Comparing base (45216a7) to head (6e7eba8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #266   +/-   ##
=======================================
  Coverage   75.73%   75.73%           
=======================================
  Files           1        1           
  Lines        1092     1092           
  Branches      342      342           
=======================================
  Hits          827      827           
  Misses         59       59           
  Partials      206      206           

Flag	Coverage Δ
perl	75.73% <ø> (ø)
xs	75.73% <ø> (ø)

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