Fix STR() truncating concatenated strings

[zheliu2]

Summary

Fixes #4297

The STR() function was passing length + 1 (buffer size including null terminator) as the string length to tsql_varchar_input(), causing the resulting varchar to include an extra byte. This extra byte caused any strings concatenated after STR() to be silently lost.

Root cause: In contrib/babelfishpg_tsql/src/string.c, the float_str() function allocated size = length + 1 bytes for the buffer (to accommodate the null terminator), but then passed size (instead of the actual string length) to return_varchar_pointer().

Fix: Changed return_varchar_pointer(buf, size) to return_varchar_pointer(buf, strlen(buf)) so the actual string length is used.

Example:

-- Before fix: returns 'distance: 123' (suffix lost)
-- After fix:  returns 'distance: 123 mi' (correct)
PRINT 'distance:' + STR(123, 4, 0) + ' mi'

Test plan

• Added test cases for string concatenation with STR() result in test/JDBC/input/functions/string_functions/str.sql
• Updated expected output in test/JDBC/expected/str.out