feat: Implement comprehensive APT solver for debian-forge

- Add complete APT solver implementation (osbuild/solver/apt.py)
- Implement Solver interface with dump(), depsolve(), search() methods
- Add package info and dependency resolution capabilities
- Support for multiple repositories with GPG key validation
- Repository priority and component filtering
- Proxy support for enterprise environments
- Root directory support for chroot environments
- Comprehensive error handling and validation
- Create extensive test suite (test/test_apt_solver*.py)
- Update solver __init__.py with graceful dependency handling
- Add comprehensive documentation (docs/apt-solver-implementation.md)

This provides native Debian package management capabilities that
are not available in upstream osbuild, making debian-forge a true
Debian-native image building solution.

Closes: APT solver implementation
Status: PRODUCTION READY

osbuild/solver/__init__.py

@@ -84,3 +84,41 @@ def read_keys(paths, root_dir=None):
         else:
             raise GPGKeyReadError(f"unknown url scheme for gpg key: {url.scheme} ({path})")
     return keys
+
+
+# Import available solvers
+__all__ = [
+    "Solver",
+    "SolverBase", 
+    "SolverException",
+    "GPGKeyReadError",
+    "TransactionError",
+    "RepoError",
+    "NoReposError",
+    "MarkingError",
+    "DepsolveError",
+    "InvalidRequestError",
+    "modify_rootdir_path",
+    "read_keys",
+]
+
+# Try to import DNF solvers (may not be available in Debian)
+try:
+    from .dnf import DNF
+    __all__.append("DNF")
+except ImportError:
+    DNF = None
+
+try:
+    from .dnf5 import DNF5
+    __all__.append("DNF5")
+except ImportError:
+    DNF5 = None
+
+# Import APT solver (always available in debian-forge)
+try:
+    from .apt import APT
+    __all__.append("APT")
+except ImportError as e:
+    print(f"Warning: Could not import APT solver: {e}")
+    APT = None