test: cover serde attributes forwarded through another attribute

[Boshen]

Adds a regression test for a serde attribute forwarded through another attribute, as confik does:

struct Foo {
    #[confik(forward(serde(with = "humantime_serde")))]
    timeout: Duration,
}

This is the shape reported in #542. It is not a macro body — confik is a derive macro, so the attribute is a plain Attr node. It failed because collect_meta_imports only calls collect_serde_attribute when the attribute's own path is literally serde, and here it is confik.

#543 already fixes it: the token scan added in collect_token_tree runs on every attribute's token tree and matches on a serde ident anywhere in the stream, so the nested serde(...) is picked up regardless of the outer path. The existing serde_attributes_inside_macro test does not cover this path — it would keep passing if the scan were ever narrowed to the attribute path or scoped to macro bodies, silently regressing confik. This test pins that behaviour: it fails with the #543 hunk reverted (left: {}, right: {"foo"}).