Fix bounded concatenation in char arrays

[Old-Ding]

Summary

• stop rcutils_char_array_strncat() at the source string terminator when it appears before n
• calculate capacity and buffer_length from the bytes actually copied
• reject destination length arithmetic overflow before resizing or copying

Testing

• linked a focused C probe against origin/rolling: returned success with buffer_length=10 for a two-character source
• linked the same probe against this change: returned success with buffer_length=7 and 123456
• compiled src/char_array.c as C11 with Zig
• git diff --check

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