Break diags log rotation into subroutines

[JosiahWI]

This change reduces a small amount of duplication and introduces meaningful function names. I'm cleaning this up now because there are (very minor) bugs in the log rotation logic (I have not filed issues yet), and my hope is that this refactoring makes the code easier to understand and review for those bugfixes.

This change restructures conditional flow to use guard clauses instead of nesting all the logic in conditional checks.

• Extract Diags::roll_diags_on_filesize

• Extract Diags::roll_diags_on_interval

• Extract Diags::roll_streams_on_filesize

• Extract Diags::roll_streams_on_interval

• Extract Diags::remake_logfile

• Extract Diags::swap_diagslog

[JosiahWI]

Instead of asserting that this flag was set, I am asserting at an earlier point that the condition for setting the flag is met. The code was originally designed to be a little easier to change if more output stream flexibility was needed. Since that obviously hasn't happened for a long time, and was only a small amount of code, I have removed the extra logic flow and this flag entirely, simplifying the code.

[Copilot]

Pull request overview

This PR refactors diagnostic and output log rotation in Diags by extracting the previously inlined rotation logic into named helper subroutines, aiming to reduce duplication and make upcoming bugfixes easier to review.

Changes:

• Extracted size-based and time-based rotation logic for diags.log into roll_diags_on_filesize() / roll_diags_on_interval().
• Extracted size-based and time-based rotation logic for stdout/stderr output logs into roll_streams_on_filesize() / roll_streams_on_interval().
• Centralized logfile recreation / swapping logic via remake_logfile() and swap_diagslog(), using std::unique_ptr for safer ownership.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/tscore/Diags.cc	Refactors log rotation and reseating logic into helpers; introduces unique_ptr management and guard-clause flow.
include/tscore/DiagsTypes.h	Declares the newly extracted private helper methods on Diags.