fix array end handling for first variant alternative in parse_into

[Ramya-9353]

Repro: parse_into a std::vector<variant2::variant<std::vector<int>, int>> from [[1],2]; it fails with error::not_array, while the alternatives-swapped variant<int, std::vector<int>> accepts the same document. A variant<std::array<int,3>> parsed from [1] reports success and leaves two elements unwritten; nesting such a variant in a map trips the BOOST_ASSERT(false) path in handler_error_base::on_object_end on debug builds.
Cause: the variant handler's on_array_end tests its int sentinel with !inner_active_, which is only true for index 0, so while the first alternative is mid-array its closing bracket is routed to signal_end and terminates the variant instead of reaching the active alternative. The completeness check of the aborted alternative never runs.
Fix: compare inner_active_ < 0, matching the sentinel checks elsewhere in the file. The added tests fail before the change and pass after.

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https://claude.ai/code/session_01C7uBZYRF8M5U3w49QLn4nW

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Build time: 2026-09-04 18:31:48 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.69%. Comparing base (82398f9) to head (a730f74).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1195      +/-   ##
===========================================
- Coverage    93.72%   93.69%   -0.03%     
===========================================
  Files           85       85              
  Lines         8981     8981              
===========================================
- Hits          8417     8415       -2     
- Misses         564      566       +2     

Files with missing lines	Coverage Δ
include/boost/json/detail/parse_into.hpp	99.00% <100.00%> (-0.34%)	⬇️

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