refactor: 将 adapter 层的计费/模型抓取/行为变体能力下沉到 core.api_format 注册表

- 新增 core/api_format/capabilities.py，统一注册计费模板、模型抓取、
  total_input_context 计算和 provider behavior variant
- 新增 core/usage_tokens.py，抽取 cache token 解析逻辑到 core 层
- handler adapter 移除各自的 compute_total_input_context / fetch_models /
  BILLING_TEMPLATE 覆盖，改为委托 core 注册表解析
- provider/behavior.py 改为薄封装，底层委托 core registry
- 新增 tests/test_architecture_import_rules.py 架构导入约束测试
- 新增 tests/services/api_format/test_capabilities.py 能力注册表测试

Closes #207

Co-authored-by: AAEE86 <ppk0227@hotmail.com>

tests/test_architecture_import_rules.py

@@ -0,0 +1,78 @@
+from __future__ import annotations
+
+import ast
+from dataclasses import dataclass
+from pathlib import Path
+
+
+@dataclass(frozen=True, slots=True)
+class _Violation:
+    file: Path
+    line: int
+    imported: str
+
+
+def _repo_root() -> Path:
+    return Path(__file__).resolve().parents[1]
+
+
+def _iter_python_files(root: Path) -> list[Path]:
+    files: list[Path] = []
+    for path in root.rglob("*.py"):
+        if "__pycache__" in path.parts:
+            continue
+        files.append(path)
+    return files
+
+
+def _scan_imports(py_file: Path) -> list[tuple[int, str]]:
+    """返回 (lineno, module_name) 列表。"""
+    text = py_file.read_text(encoding="utf-8").lstrip("\ufeff")
+    tree = ast.parse(text, filename=str(py_file))
+
+    imports: list[tuple[int, str]] = []
+    for node in ast.walk(tree):
+        if isinstance(node, ast.Import):
+            for alias in node.names:
+                imports.append((int(getattr(node, "lineno", 0) or 0), str(alias.name)))
+        elif isinstance(node, ast.ImportFrom):
+            if node.level:
+                continue
+            if node.module:
+                imports.append((int(getattr(node, "lineno", 0) or 0), str(node.module)))
+    return imports
+
+
+def _scan_forbidden_imports(*, scope_dir: Path, forbidden_prefix: str) -> list[_Violation]:
+    violations: list[_Violation] = []
+    for py_file in _iter_python_files(scope_dir):
+        for line, imported in _scan_imports(py_file):
+            if imported == forbidden_prefix or imported.startswith(f"{forbidden_prefix}."):
+                violations.append(_Violation(file=py_file, line=line, imported=imported))
+    return violations
+
+
+def _format_violations(title: str, violations: list[_Violation]) -> str:
+    lines = [title, ""]
+    for v in sorted(violations, key=lambda x: (str(x.file), x.line, x.imported)):
+        rel = v.file.resolve().relative_to(_repo_root())
+        lines.append(f"- {rel}:{v.line} -> {v.imported}")
+    return "\n".join(lines)
+
+
+def test_services_should_not_import_api() -> None:
+    repo = _repo_root()
+    violations = _scan_forbidden_imports(
+        scope_dir=repo / "src" / "services",
+        forbidden_prefix="src.api",
+    )
+    assert not violations, _format_violations("services 层禁止 import api 层：", violations)
+
+
+def test_core_should_not_import_services() -> None:
+    repo = _repo_root()
+    violations = _scan_forbidden_imports(
+        scope_dir=repo / "src" / "core",
+        forbidden_prefix="src.services",
+    )
+    assert not violations, _format_violations("core 层禁止 import services 层：", violations)