🧪 Add unit tests for is_deferred concept and trait

[ipapadop]

🧪 [Add unit tests for is_deferred concept and trait]

🎯 What:

The is_deferred concept and trait were missing dedicated unit tests. This gap meant that the core mechanism for identifying deferred expressions wasn't explicitly verified.

📊 Coverage:

The new tests in test/unit/is_deferred.cpp cover:

• Positive cases: constant_, variable_, and if_then_else_expression (the primary building blocks of the library).
• Negative cases: Standard types like int, std::vector, and custom dummy types.
• Type Qualifications: Ensured Deferred and is_deferred correctly handle const, &, const &, and && by using std::remove_cvref_t.
• AnyDeferred Concept: Verified that the variadic concept correctly identifies when at least one type in a pack is deferred.

✨ Result:

Improved the reliability of the library's type-dispatching system by ensuring the fundamental Deferred concept behaves as expected across various type combinations. All tests were verified via static analysis and compile-time assertions.

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PR created automatically by Jules for task 6535957170840422710 started by @ipapadop

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[gemini-code-assist]

Code Review

This pull request registers and adds a new unit test file test/unit/is_deferred.cpp to verify the is_deferred trait, Deferred concept, and AnyDeferred concept. The feedback suggests explicitly including the <type_traits> header to follow the "Include What You Use" principle, adding test coverage for volatile and const volatile cv-qualifications, and adding a test case for the empty template parameter pack case of AnyDeferred<>.