bindings-macro: reject non-ident reducer/procedure/view argument patterns (#2369)

[Manojseetaram]

Description of Changes

API and ABI breaking changes

Expected complexity level and risk

Testing

[Manojseetaram]

The bug

crates/bindings-macro's reducer, procedure, and view macros extract each
argument's name for the ARG_NAMES const, which feeds into the schema
(ProductType) sent to SDK/client codegen. When an argument's pattern
wasn't a plain identifier (e.g. a destructuring pattern like
SomeProduct { a, b }: SomeProduct), the macro silently emitted None
instead of erroring — producing an unnamed field in the generated schema
instead of any indication something was wrong.

The fix

Reject non-identifier argument patterns at macro-expand time with a clear
error, rather than the alternative (auto-generating synthetic names) — this
seemed like the safer, simpler option to land first. Bare _ wildcards are
still allowed, since _ intentionally means "no name" and doesn't lose any
named sub-field information the way destructuring does.

This logic was duplicated identically across reducer.rs, procedure.rs,
and view.rs, so I extracted it into a shared extract_arg_names helper in
util.rs.
x

​```rust
#[derive(SpacetimeType)]
struct SomeProduct {
a: i32,
b: i32,
}

#[spacetimedb::reducer]
fn foo(_: &ReducerContext, _: i32, SomeProduct { a, b }: SomeProduct) {
let _ = a + b;
}
​```

cargo check -p sdk-test-module now fails with:

​ error: arguments must be simple identifiers (e.g. `x: MyType`) or `_`, not patterns like destructuring; bind to a name and destructure in the function body instead --> modules/sdk-test/src/lib.rs:842:36 | 842 | fn foo(_: &ReducerContext, _: i32, SomeProduct { a, b }: SomeProduct) { | ^^^^^^^^^^^^^^^^^^^^ ​

Confirmed all other existing reducers/procedures/views in the repo still
compile (searched for any _ wildcard usage as an argument — none exist
currently, but the fix explicitly allows it going forward).

[bfops]

Hi @Manojseetaram, if you're intending for us to take a look at this PR, please fill out the description.