chore(deps): update syn requirement from 2 to 3

Updates the requirements on [syn](https://github.com/dtolnay/syn) to permit the latest version.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@2.0.0...3.0.3)

---
updated-dependencies:
- dependency-name: syn
  dependency-version: 3.0.3
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

crates/rmcp-macros/Cargo.toml

@@ -16,7 +16,7 @@ documentation = "https://docs.rs/rmcp-macros"
 proc-macro = true
 
 [dependencies]
-syn = {version = "2", features = ["full"]}
+syn = {version = "3", features = ["full"]}
 quote = "1"
 proc-macro2 = "1"
 serde_json = "1.0"