fix: provide the test stubs for pg_sys extern statics via pg_module_magic! macro

[peterkyle01]

Fixes #2281

A pgrx extension is compiled as a .so or .dylib that gets dlopen'd into PostgreSQL at runtime. All the PostgreSQL symbols like CurrentMemoryContext and BufferBlocks are provided by the running PostgreSQL process so the .so doesn't need to define them.

When cargo test runs, Rust produces a standalone test executable which is not loaded into PostgreSQL. Those symbols don't exist anywhere so the linker fails with undefined symbol.

Normally --gc-sections prunes all unreferenced pg_sys extern declarations. But #[typetag::serde] uses inventory::submit!() to register trait implementations via a "life before main" constructor and that constructor unconditionally references the trait impl methods, which reference pg_sys symbols, pulling them into the live-code set and defeating dead code elimination.

In this PR:

The fix adds #[unsafe(no_mangle)] stub definitions for the commonly-leaked pg_sys extern statics CurrentMemoryContext, error_context_stack, PG_exception_stack, emit_log_hook, Log_error_verbosity, Log_line_prefix, Log_destination, Log_destination_string, syslog_sequence_numbers, syslog_split_messages and BufferBlocks inside the pg_magic_func!() macro, gated behind #[cfg(test)].

Why the macro?

• #[cfg(test)] only fires in the crate under test so dependencies like pgrx-pg-sys are never compiled with cfg(test) by Cargo so placing the stubs in the macro ensures they expand in the extension's own source, where cfg(test) actually works.
• Every extension already calls pg_module_magic!()
• The .so build never includes the stubs (no cfg(test)) so PostgreSQL's real symbols are never shadowed.