[solvers/symex] Add ConcolicInputSpec and SymExOptions to CFG symbolic execution engine.

[copybara-service]

[solvers/symex] Add ConcolicInputSpec and SymExOptions to CFG symbolic execution engine.

Adds ConcolicInputSpec and SymExOptions to configure symbolic path
exploration. ConcolicInputSpec supports binding concrete parameter values
programmatically. SymExOptions bundles concolic inputs with
path exploration limits (max_paths), enabling upfront branch pruning
and preventing path explosion.

• ConcolicInputSpec (concolic_input_spec.{h,cc}): Encapsulates concrete
  parameter assignments with fluent builder methods.
• SymExOptions (cfg_symex_engine.h): Configuration struct for ExplorePaths
  holding concrete parameter specifications and optional max_paths limit.
• CfgSymExEngine (cfg_symex_engine.{h,cc}): Uses SymExOptions to bind
  concrete constants and enforce path limits during forward DFS traversal.
• Tests:
  • concolic_input_spec_test.cc: Unit tests for ConcolicInputSpec construction
    and parameter querying.
  • cfg_symex_engine_test.cc: Unit tests verifying concolic branch pruning and
    max_paths limit.
  • symex_e2e_test.cc: Integration test demonstrating ALU path pruning with
    concrete opcode.