fix: support qualified types in topic handlers

[immanuwell]

Description

#[topic] blows up if the handler arg uses a qualified type like serde_json::Value.

The macro was splitting tokens on : so serde_json::Value got treated like 2 inputs. This switches it to parse the function signature with syn, keeps the String fast path, and adds regression tests. tiny fix, but pretty real.

Repro:

#[topic(pub_sub_name = "pubsub", topic = "A")]
async fn handle_a_event(order: serde_json::Value) {}

Before:
cargo check panics with Expected to only have one input variable

After:
cargo check passes.

Issue reference

This PR will close #172

Checklist

• Code compiles correctly
• Created/updated tests
• Extended the documentation