Centralize action-argument validation in modelapi

[wshlavacek]

Summary

Route `Action.init` (in `structs.py`) through a single `ActionList.validate_action` method (added to `utils.py`) so that direct construction of `Action` instances applies the same schema check as the parser.

The previous inline validation in `Action.init` had two bugs:

• For positional actions (`no_setter_syntax` / `square_braces`, e.g. `setConcentration`, `quit`, `saveConcentrations`) the `arg_dict[type]` lookup returns an empty list `[]`, so the `len(valid_arg_list) > 0` guard skipped validation entirely — any args were accepted.
• For unknown action types the error message was good, but it ran AFTER setting `self.type`, leaving a half-constructed object alive in the rare case someone catches the exception.

What the new validation enforces

• Positional actions store args as a dict with `None` values (the canonical shape the parser emits). Keyword-shape dicts passed by hand are rejected with a clear error pointing at the canonical form.
• Per-action positional arity (`positional_arity` table) catches `Action("quit", {'"x"': None})` (too many) and `Action("setConcentration", {'"A()"': None})` (too few).
• Normal-type validation: arg names must be in `arg_dict[type]` (existing behavior, just centralized).

No behavior change at parse time

The parser already emits the canonical positional-args-as-`{value: None}` shape, so the stricter direct-construction path doesn't reject anything the parser produces. Only direct `Action(...)` calls with malformed args see a new error (which is the point).

Test plan

• Existing CI passes
• `Action("simulate", {"method": '"ode"'})` constructs as before
• `Action("setConcentration", {'"A()"': None, "100": None})` constructs and gen_strings to `setConcentration("A()",100)`
• `Action("setConcentration", {"A()": "100"})` raises `BNGParseError` (the bug fix)