Fix mypy arg-type errors in generated discriminated union encoders

[darshkpatel]

Why

The codegen for discriminated union TypedDict encoders produces ternary chains like:

encode_Foo(x) if x["kind"] == "foo" else encode_Bar(x)

mypy can't narrow union types through these ternary conditions, so it flags every encoder call as receiving the wrong type (arg-type).

This broke the pid2 codegen CI when new discriminated union variants were added to a schema.

What changed

Use cast() to explicitly narrow the type to the correct variant after the discriminator check, instead of suppressing with # type: ignore[arg-type]. This preserves type safety in the generated code.

Before:

encode_Foo(x)  # type: ignore[arg-type]
if x["kind"] == "foo"
else encode_Bar(x)  # type: ignore[arg-type]

After:

encode_Foo(cast('Foo', x))
if x["kind"] == "foo"
else encode_Bar(cast('Bar', x))

Affects both the single-variant and multi-variant discriminator code paths.

Test plan

CI