test_pyo3_wrapper: _link_libpython_wrapper skips on any build failure, not only a missing libpython

[terasakisatoshi]

Found by Codex on PR #334 (docs). _link_libpython_wrapper in test/test_pyo3_wrapper.jl (lines ~42–54) wraps link planning, the crate scan and build_pyo3_wrapper in one try … catch and turns any exception into @info "skipping the :link_libpython wrapper testset". The intent is to skip on machines without a linkable libpython3.x; the effect is that a wrapper code-generation, Cargo or compiler regression also reports as a skip — on Ubuntu CI too, where a linkable Python exists — so the suite can go green without having built a :link_libpython wrapper.

Proposed fix

• Decide the prerequisite up front: compute pyo3_link_plan(crate) and check python_link_source() / the plan's library directory; skip only when plan.mode !== :link_libpython or the directory holds no linkable libpython3.x (the same check the plan itself performs before refusing).
• Once the prerequisite holds, let build_pyo3_wrapper failures fail the testset (no catch).
• Reuse the helper from the PyO3 cross-module case of test/test_module_symbols.jl (Exported symbols carry no module path: rustcall_<name> collides across modules (#279 scheme) #300 / PR Module-qualified exported symbols: #[julia] on modules, module_path in crate mode, schema 7 (#300) #333) so both gate the same way.
• Keep the @info for the genuine skip, with the reason (no interpreter / no shared library / plan mode).

Acceptance

• a forced wrapper build failure on a machine with a linkable Python fails test_pyo3_wrapper.jl instead of skipping;
• on a machine without a linkable Python the testsets still skip with an informative message;
• docs/src/pyo3.md "What a :link_libpython build needs from the machine" and CLAUDE.md are updated to say the skip is now the prerequisite check only.

[terasakisatoshi]

Also route the mixed-crate testset ("a mixed crate keeps its #[julia] items (#307 review)", test/test_pyo3_wrapper.jl ~455–492) through the same helper: today it has its own try that logs @info "skipping the mixed-crate build" and records a @test_skip, so it is a second broad skip path with a different message (Codex on #334).

[terasakisatoshi]

Two more broad skip paths in test/test_pyo3_wrapper.jl to route through the shared prerequisite check: the feature-gated build (@info "skipping the feature-gated build", ~lines 537–545) and the configured-crate build (@info "skipping the configured-crate build", ~lines 710–719). Four messages in total today; the docs (#334) tell readers to grep a run for "skipping".