fix: type-conversion edge cases in TypeTools and Option reduce

[henryiii]

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Fixes five verified type-conversion bugs in TypeTools.hpp and Option_inl.hpp, each with a regression test.

1. Unsigned integral_conversion accepted " -1" as UINT64_MAX. The old guard only checked input.front() == '-', but strtoull skips leading whitespace and silently wraps negatives. Now the input is rejected when its first non-whitespace character is '-'. (include/CLI/TypeTools.hpp)

2. Float lexical_cast converted whitespace-only strings to 0.0 successfully. When strtold performs no conversion the end pointer equals the start; the trailing-whitespace skip loop then walked to the end and returned true. Now we return false when nothing was consumed (val == input.c_str()). (include/CLI/TypeTools.hpp)

3. Uninitialized v2 could be inserted into a container-of-pairs (UB). With an odd element count the second conversion was skipped while retval stayed true, inserting {v1, v2} with v2 never assigned. Both values are now value-initialized and a missing second element sets retval = false, so it raises a ConversionError instead of silently inserting {key, 0}. Reachable via add_result("onlykey"); results(map);. (include/CLI/TypeTools.hpp)

4. sum_string_vector broke for integer sums >= 1e16 and lost precision above 2^53. Sums accumulated as double and printed at precision 16, rendering large totals in scientific notation that integral_conversion could not parse. An int64_t accumulation (with overflow check) is now tried first, falling back to the existing double path for floats/flags/non-numeric input. (include/CLI/TypeTools.hpp)

5. Join policy in _reduce_results tested results_.size() instead of original.size(). Option::results(T&) reduces results synthesized from default_str_ while results_ is empty, so the join was skipped (yielding "a" instead of "a\nb"). Now tests original.size() > 1 like every other policy branch. (include/CLI/impl/Option_inl.hpp)

No false positives — all five reproduced against the code before the fix.

Tests added in tests/HelpersTest.cpp (unsigned leading-whitespace negative, whitespace-only float, odd-count pair container, sum_string_vector) and tests/OptionTypeTest.cpp (Join + default_str via results(), large-integer Sum policy).

Part of #1357