[NopeCharon]

Summary: Fix parsing of /proc/<pid>/stat when the command name contains whitespace

The command name (comm) field of /proc/<pid>/stat is wrapped in parentheses and can contain any character, including runs of spaces, newlines and parentheses. ProcParser::ParseProcPIDStat() and GetPIDStartTimeTicks() read only the first line, split it with absl::SkipWhitespace() and then shifted every field index by the number of spaces in the name. As described in the issue, this goes wrong when:

• the name has consecutive spaces: they are collapsed by the split but each one is counted in the offset, so later fields are read from the wrong column;
• the name has a newline: the record is truncated and the field-count check fails;
• the field-count check itself does not account for spaces in the name.

Since GetPIDStartTimeTicks() feeds UPID construction, such processes could end up with a wrong start time or be skipped.

This PR adds a SplitProcPIDStat() helper that reads the whole file, takes the command name as everything between the first ( and the last ) (no later field can contain a )), and only splits the remainder on whitespace. The returned fields line up with the existing kProcStat*Field constants, so the offset arithmetic is gone. Both parsers now use it.

In the ASAN-only path of GetPIDStartTimeTicks(), the file is now read with fread() instead of fgets() so names with newlines work there too, the buffer is trimmed to the bytes actually read, and the FILE* (previously leaked) is closed.

Relevant Issues: Fixes #678

Type of change: /kind bug

Test Plan: Added parameterized tests to //src/common/system:proc_parser_test that write a stat file with names containing consecutive, leading and trailing spaces, newlines, unbalanced parentheses and an empty name, and check every parsed field for both ParseProcPIDStat() and GetPIDStartTimeTicks(). I also ran the new splitting logic in a standalone program on Linux against the real /proc/<pid>/stat of child processes renamed with prctl(PR_SET_NAME) (a b, a\nb, x\n\n y, a) (b, empty, etc.). The new logic returned the correct pid, comm, thread count and start time for all of them, while the previous logic returned a wrong start time for names with consecutive spaces or newlines. clang-format and cpplint are clean on the changed files.

Changelog Message:

Fixed process stats and UPID start time parsing for processes whose name contains consecutive spaces or newlines.