fix(secrets): complete the binding and value contract at the Rust boundary

`validate` checked only the binding count, so a caller reaching the low-level
entry point past `bind_secrets` could register an environment variable name the
runtime cannot deliver, or bind a Secret to a name `runtime.env` already sets --
which would resolve by delivery order, with a value visible in the Function's
record and a value that is not.

Secret values are bounded here too, at the limit the service enforces, so an
oversized credential is refused before a request body is built rather than
after it has been serialized and uploaded.

Sandbox-reserved names are deliberately still the service's alone: that list
belongs to the runtime that owns it, and a copy here would drift from it
silently.

Both gate reproducers now fail closed with no request reaching the service.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UfmeJ533rQDnPBkMtjerV6

python/python/tests/test_first_class_function_slice2.py

@@ -179,6 +179,35 @@ def test_a_function_binds_at_most_sixteen_secrets():
     assert state["requests"] == []
 
 
+def test_binding_names_are_validated_below_the_python_api():
+    """The low-level entry point reaches the same validator the typed API does.
+
+    Registration envelopes can be hand-rolled past ``bind_secrets``, so the
+    grammar and the disjointness rule live in Rust, above the backend.
+    """
+    with _mock_remote_function_catalog() as (host, state):
+        db = lancedb.connect(
+            "db://dev",
+            api_key="fake",
+            host_override=host,
+            client_config={"retry_config": {"retries": 0}},
+        )
+        envelope = json.loads(analyze_caption.registration_request.to_canonical_json())
+        envelope["secret_env_bindings"] = {
+            "BAD=NAME": "openai-prod",
+            "TOKEN_0": "secret-0",
+        }
+        envelope["runtime"]["env"]["TOKEN_0"] = "public"
+
+        async def submit_envelope():
+            return await db._conn._inner.create_function_async(json.dumps(envelope))
+
+        with pytest.raises(ValueError, match="portable"):
+            LOOP.run(submit_envelope())
+
+    assert state["requests"] == []
+
+
 _SECRET_DEBUG_LOG_SOURCE = """
 import http.server
 import json