fix: decouple libidalib from plugin build

[0xeb]

Summary

• Decouples libidalib from the IDA plugin binary. The static library (liblibidasql.a) no longer links idasdk::idalib — it only uses compilation-only targets (ida_compiler_settings, ida_platform_settings) plus SDK include paths. Each consumer brings its own SDK target.
• Extracts Session implementation into session.cpp. Session is the only code with idalib symbol references (init_library, open_database, close_database). When the plugin links the static archive, session.o is never pulled in — the idalib symbols never need resolution.
• No header or API changes. The header still declares Session (declarations don't create symbol references). CLI and plugin CMakeLists are unchanged — they already get the right SDK target from ida_add_idalib() / ida_add_plugin().

Before

All three platform plugin binaries (.dll, .so, .dylib) unnecessarily linked libidalib.

After

• Plugin: links libida.dylib only — no libidalib
• CLI: links both libida.dylib and libidalib.dylib (as expected)

Verified locally on macOS:

$ otool -L idasql.dylib | grep idalib
# (no output — libidalib is gone)

$ otool -L idasql | grep idalib
@rpath/libidalib.dylib

Test plan

• CI builds pass on all platforms (Windows, Linux, macOS)
• Plugin binary does NOT link libidalib on any platform
• CLI binary still links both libida and libidalib
• Plugin loads in IDA GUI and HTTP server works
• CLI: idasql -s <db> -q "SELECT * FROM welcome" succeeds