ci: add header standalone self-containment check

Summary

Add a CI job that compiles headers under include/libssh in isolation to catch include-order / self-containment regressions early.

This is meant to be a safety net: it keeps the pipeline green for known existing failures, but fails when a new standalone-header regression is introduced.

What this MR does

• adds the header-standalone CI job
• runs it after CMake configuration so generated headers are available
• checks headers under include/libssh
• uses an allowlist for currently known standalone-compilation failures
• reports details in GitLab-friendly collapsible log sections
• fails on regressions that are not already allowlisted

What this MR does not do

• it does not fix all existing failing headers
• it does not try to make every header self-contained in one step

Follow-up work

• remove allowlist entries as headers are fixed or start passing consistently
• keep the job as a regression net for future changes

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated
• The project pipelines timeout is extended at least to 2 hours.

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code