Fix exceptions from allocator-taking VLA move construction

[thirtytwobits]

An allocator-taking VLA move can allocate new storage when the allocators differ. Its unconditional noexcept currently turns an allocation failure into std::terminate, preventing the caller from catching std::bad_alloc.

Allow exceptions to propagate through the allocating base constructor and make both the generic and packed-bool public constructors' exception specifications match the existing allocator dispatch. Ordinary moves and the existing unconditional storage-transfer paths retain noexcept.

Add regression tests for allocation failure, preservation and subsequent reuse of the source, equal-allocator moves without allocation, and the exception specifications for all allocator-trait combinations. Capacity bookkeeping and partial-construction cleanup remain tracked separately in #205 and #37.

Closes #204.

Validation:

• The new compile-time regression checks fail on the original implementation for both int and bool.
• The original issue reproducer now catches std::bad_alloc and exits successfully.
• All seven VLA suites pass in C++14 under GCC 13.4 and Clang 22.1.8, in Debug and exceptions-disabled ReleaseEP configurations (existing compiler/platform skips retained).
• Both affected test translation units pass C++17, C++20, and C++23 compilation under GCC and Clang, with exceptions enabled and disabled.
• Formatting checks for the changed regions and git diff --check pass.

[Copilot]

Copilot review overview

🟡 Changes recommended

POCMA-only allocators can still transfer storage between unequal allocators, causing deallocation through the wrong allocator.

Review effort: Balanced
Findings: 1

Open (1)

• Avoid stealing storage with unequal allocator in move construction · New

What changed in this PR

Updates allocator-aware VLA move construction so allocation failures can propagate.

Changes:

• Removes unconditional noexcept from allocating moves.
• Adds allocation-failure and allocator-trait regression tests.
• Preserves no-throw ordinary and storage-transfer moves.

File	Description
include/​cetl/​variable_length_array.hpp	Adjusts move-constructor exception specifications.
cetlvast/​suites/​unittest/​test_variable_length_array_copy_and_move.cpp	Tests allocator failure and source preservation.
cetlvast/​suites/​unittest/​test_variable_length_array_compiles.cpp	Adds compile-time exception-specification checks.

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[Copilot]

Copilot review overview

🟡 Changes recommended

POCMA is incorrectly used to permit storage transfer and noexcept construction with potentially unequal stateful allocators.

Review effort: Balanced
Findings: 2

Open (2)

• Fix noexcept assertion for propagating stateful allocators · New
• Do not use POCMA for allocator-extended move noexcept · New

Resolved since last review (1)

• Avoid stealing storage with unequal allocator in move construction