Refuse an unknown run input by name before reading its value

`fluksio run --param lr=0.002` died with a bare JSONDecodeError: `--param`
is not a `run` flag, so it became an input named `param` whose value
`lr=0.002` was json-decoded. The name check ran after the coercion, so the
decode error always won. Names are now checked first, and a coercion error
is a SyncError naming the input and its type, the way `_ask_params` has
always done it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Hra4ndWMCLU5F3KjUuVAc

backend/tests/test_cli.py

@@ -88,6 +88,24 @@ def test_run_arguments_are_typed_by_the_flow_they_are_for() -> None:
         _params(definition, ["--nonesuch", "1"])
 
 
+def test_a_sweeps_param_spelling_is_refused_by_name() -> None:
+    """`run --param lr=0.002` is a name this flow has not got, and says so."""
+    import pytest
+
+    from fluksio.sdk import SyncError
+    from fluksio.sdk.cli import _params
+
+    definition = {"inputs": [{"spec": {"name": "lr", "dtype": "float"}}]}
+
+    # Not a JSONDecodeError over `lr=0.002`, which is what reading the value
+    # before the name used to give.
+    with pytest.raises(SyncError, match="sweep --param"):
+        _params(definition, ["--param", "lr=0.002"])
+
+    with pytest.raises(SyncError, match="'lr' takes float"):
+        _params(definition, ["--lr", "fast"])
+
+
 def test_serve_uses_the_installation_the_directory_belongs_to(
     tmp_path: Path, monkeypatch
 ) -> None: