Skip symbol check when tools are unavailable

[ronag]

Summary

• skip the unresolved-symbol test when nm is not installed
• skip the same test when c++filt is not installed
• continue to fail for other process-launch errors, nonzero tool exits, and unresolved vendored symbols

Why

This follows up on the post-merge review comment on #2. The test assumed both binutils-style tools were present on every non-Windows system, so otherwise valid CI images and developer machines could fail for an environmental reason unrelated to the addon.

Impact

The native symbol validation remains strict wherever both tools are available. Environments missing either optional diagnostic tool now receive an explicit test skip instead of a hard failure.

Validation

• Node 26.5.0 on macOS arm64: full native rebuild, 14/14 runtime tests, and TypeScript tests passed
• empty PATH: symbol test skipped with nm is not available
• isolated PATH containing only nm: symbol test skipped with c++filt is not available