src: use constexpr table for GetOctalCode

[ansh1406]

Replaced the runtime std::vector<std::string> cache used by GetOctalCode() with a compile-time constexpr lookup table of std::string_view.

Changes

1. GetOctalCode() now returns std::string_view instead of const std::string&.

2. There are two call sites that use GetOctalCode():

   • src/node_snapshotable.cc: The returned value is streamed directly to an std::ostream, so no changes were required.
   • tools/js2c.cc: Updated the implementation to use std::string_view::data() instead of std::string::c_str().

Verification

• Verified that the generated output is identical for all 256 byte values.
• Full build completed successfully on Debian 13.4 (x86_64, GCC, make).