doctest: honor SKIP flags in testcode blocks

[Shashankrawat1504]

Description

Fixes an issue where # doctest: +SKIP directives in testcode blocks were ignored.

Previously, inline doctest flags in testcode source were not interpreted when the code was executed together with a paired testoutput block. As a result, statements marked with +SKIP were still executed.

This change:

• honors statement-level doctest flags in testcode blocks;
• preserves the existing paired testcode/testoutput execution model;
• keeps regular doctest block behavior unchanged;
• preserves the existing trim_doctest_flags behavior;
• adds regression tests for +SKIP and other doctest flags;
• documents the supported behavior.

Testing

Relevant doctest and transform test suites pass, along with Ruff, Sphinx lint, formatting checks, and git diff --check.