fix (str2num) : handle leading '+' sign in to_num parsers

[jalvesz]

Fix #1197 by CoPilot:

to_num() silently returned 0 for any numeric string with a leading + (e.g. "+1", "+1.234"). The integer and all four real parsers (to_sp_base, to_dp_base, to_xdp_base, to_qp_base) only consumed a leading -; a + was left unadvanced, the digit loop found a non-digit immediately, and the parser succeeded with value 0.

k = to_num("+1", k)   ! returned 0 (wrong)
k = to_num("-1", k)   ! returned -1 (correct)

Changes:

• src/strings/stdlib_str2num.fypp — in all five to_*_base parsers, extended the leading-sign check from an if (minus only) to if / else if that also consumes a leading + and advances p. Updated the comment from "Verify leading negative" to "Verify leading sign".
• test/string/test_string_to_number.fypp — added regression cases for integers ("+1", "+42", " +99", "+0005") and reals ("+1", "+1.234") across all type variants.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.62%. Comparing base (478ef58) to head (26fc53c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1198      +/-   ##
==========================================
+ Coverage   68.60%   68.62%   +0.01%     
==========================================
  Files         409      409              
  Lines       13784    13784              
  Branches     1559     1559              
==========================================
+ Hits         9457     9459       +2     
+ Misses       4327     4325       -2     

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