refactor: extract transport-agnostic core and shared tool registry

Introduce aegis_gitea_mcp.registry as the single name->handler source of
truth consumed by every transport adapter, moving TOOL_HANDLERS out of the
FastAPI server module. Add aegis_gitea_mcp.errors.ToolError so core handlers
no longer import fastapi.HTTPException; raw_tools now raises ToolError and the
HTTP adapter maps it back to HTTPException, preserving status codes and audit
behavior. Add a subprocess boundary test asserting the core imports without
pulling in fastapi/uvicorn/starlette.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_core_boundary.py

@@ -0,0 +1,64 @@
+"""Lock the transport-agnostic core boundary.
+
+The core (tool registry, Gitea client, policy, audit, config, request context,
+tools) must import cleanly without dragging in the web stack. If a stray
+``import fastapi`` creeps back into a core module, the local stdio package would
+gain a needless heavy dependency and the ``[server]`` extra split would leak.
+
+The check runs in a subprocess because, within the pytest process, FastAPI is
+already imported by the server tests — so ``'fastapi' in sys.modules`` would be
+true regardless. A clean interpreter is the only reliable probe.
+"""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+_SRC = Path(__file__).resolve().parents[1] / "src"
+
+# Core modules that must stay free of the web stack.
+_CORE_MODULES = [
+    "aegis_gitea_mcp.registry",
+    "aegis_gitea_mcp.gitea_client",
+    "aegis_gitea_mcp.policy",
+    "aegis_gitea_mcp.audit",
+    "aegis_gitea_mcp.config",
+    "aegis_gitea_mcp.request_context",
+    "aegis_gitea_mcp.response_limits",
+    "aegis_gitea_mcp.security",
+    "aegis_gitea_mcp.cache",
+    "aegis_gitea_mcp.logging_utils",
+    "aegis_gitea_mcp.mcp_protocol",
+    "aegis_gitea_mcp.errors",
+    "aegis_gitea_mcp.tools.raw_tools",
+    "aegis_gitea_mcp.tools.read_tools",
+    "aegis_gitea_mcp.tools.write_tools",
+    "aegis_gitea_mcp.tools.repository",
+    "aegis_gitea_mcp.tools.arguments",
+]
+
+
+def test_core_does_not_import_fastapi() -> None:
+    """Importing the core in a clean interpreter must not import FastAPI."""
+    imports = "\n".join(f"import {module}" for module in _CORE_MODULES)
+    program = (
+        f"import sys\n{imports}\n"
+        "leaked = [m for m in ('fastapi', 'uvicorn', 'starlette') if m in sys.modules]\n"
+        "assert not leaked, f'core leaked web stack: {leaked}'\n"
+        "print('ok')\n"
+    )
+    env = dict(os.environ)
+    env["PYTHONPATH"] = str(_SRC)
+    result = subprocess.run(
+        [sys.executable, "-c", program],
+        env=env,
+        capture_output=True,
+        text=True,
+    )
+    assert result.returncode == 0, (
+        f"core import boundary violated.\nstdout: {result.stdout}\nstderr: {result.stderr}"
+    )
+    assert "ok" in result.stdout