refactor: remove unused exception classes

[kingpy-bot]

Summary

• remove four exception classes that are never referenced outside their own declarations
• keep the exception namespace limited to the three error types the package actually throws
• reduce misleading public API surface left over from the pre-v3 design

Verification

• full Pest suite: 103 tests, 259 assertions
• Laravel Pint check: 45 files passed
• git diff --check
• repository-wide search confirms the four removed class names have no remaining references

Closes #99.