ffi: allocate string argument storage lazily - #66178

InvokeFunction reserves a temporary std::vector<std::string> on every call with arguments, even when none of the arguments are strings. Defer that reservation until the first string argument needs to be copied, avoiding an allocation and deallocation for calls without string arguments.

The first string still reserves capacity for the full argument count before its pointer is saved. Subsequent strings therefore cannot reallocate the vector and invalidate pointers already prepared for the native call.

Add a test that exercises native argument conversion with multiple strings, empty and long strings, Unicode, mixed string/Buffer arguments, and errors while converting a later argument.

Local macOS ARM64 comparison using benchmark/compare.js, with 10 samples per binary and the default iteration counts across all 40 FFI benchmark configurations:

                                                              confidence   improvement   accuracy (*)    (**)   (***)
ffi/many-args.js n=10000000                                   ***             +5.97 %   ±2.33%  ±3.26%  ±4.60%
ffi/sum-5-i32.js n=10000000                                   ***            +11.18 %   ±2.49%  ±3.42%  ±4.67%

Both improvements remained significant at the 5% level after the runner's Holm-Bonferroni correction. The other 38 configurations showed no statistically significant difference in this run.

Validation:

• Release build with Ninja (--without-node-snapshot --without-node-code-cache).
• All 16 FFI test files passed, including the new test.
• C++ lint, import checks, and ESLint for the new test passed.

The full Node.js test suite and cross-platform CI have not been run.