Add some API parameter validation

Where it matters: when misuse would raise confusing errors.

(Assisted by Claude Code; any errors are mine.)

Author: marktsuchida

src/cjdk/_api.py

@@ -331,6 +331,9 @@ def cache_file(
     The check for SHA-1/SHA-256/SHA-512 hashes is only performed after a
     download; it is not performed if the file already exists in the cache.
     """
+    _conf.check_str("name", name)
+    _conf.check_str("url", url, allow_empty=False)
+    _conf.check_str("filename", filename, allow_empty=False)
     if ttl is None:
         ttl = 2**63
     check_hashes = _make_hash_checker(
@@ -397,6 +400,8 @@ def cache_package(
     unextracted archive) after a download; it is not performed if the directory
     already exists in the cache.
     """
+    _conf.check_str("name", name)
+    _conf.check_str("url", url, allow_empty=False)
     check_hashes = _make_hash_checker(
         dict(sha1=sha1, sha256=sha256, sha512=sha512)
     )

src/cjdk/_conf.py

@@ -51,9 +51,29 @@ class Configuration:
     _allow_insecure_for_testing: bool
 
 
+def check_str(
+    name: str,
+    value: object,
+    *,
+    allow_none: bool = False,
+    allow_empty: bool = True,
+) -> None:
+    if value is None:
+        if allow_none:
+            return
+        raise TypeError(f"{name} must be a string, got None")
+    if not isinstance(value, str):
+        raise TypeError(f"{name} must be a string, got {type(value).__name__}")
+    if not allow_empty and value == "":
+        raise ConfigError(f"{name} must not be empty")
+
+
 def configure(**kwargs: Unpack[ConfigKwargs]) -> Configuration:
     # kwargs must have API-specific items removed before passing here.
 
+    for name in ("jdk", "os", "arch", "vendor", "version"):
+        check_str(name, kwargs.get(name), allow_none=True)
+
     jdk = kwargs.pop("jdk", None)
     if jdk:
         if kwargs.get("vendor"):