fix(data-contracts): compile linkable tests on no_std and add them to the test matrix

The #[cfg(test)] module in linkable.rs relied on the std prelude for
String/Vec/ToString, so 'cargo test -p aimdb-data-contracts
--no-default-features --features linkable' failed to compile (found by
@thaodt in #155). Add the alloc imports.

The gap went unnoticed because 'make check' never compiled that module:
'linkable' appeared only in build (no test code), clippy without
--all-targets (tests not linted), and test-embedded (cargo check). Add
'linkable' to both data-contracts test matrix lines so the unit and
derive tests actually run — under std and no_std + alloc.

Refs #155

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: lxsaah

Makefile

@@ -112,9 +112,9 @@ build:
 test:
 	@printf "$(GREEN)Running all tests (valid combinations)...$(NC)\n"
 	@printf "$(YELLOW)  → Testing aimdb-data-contracts (std)$(NC)\n"
-	cargo test --package aimdb-data-contracts --features "std,simulatable,migratable,observable"
-	@printf "$(YELLOW)  → Testing aimdb-data-contracts (no_std + alloc + migratable)$(NC)\n"
-	cargo test --package aimdb-data-contracts --no-default-features --features alloc,migratable
+	cargo test --package aimdb-data-contracts --features "std,simulatable,migratable,observable,linkable"
+	@printf "$(YELLOW)  → Testing aimdb-data-contracts (no_std + alloc + linkable + migratable)$(NC)\n"
+	cargo test --package aimdb-data-contracts --no-default-features --features alloc,linkable,migratable
 	@printf "$(YELLOW)  → Testing aimdb-core (no_std + alloc)$(NC)\n"
 	cargo test --package aimdb-core --no-default-features --features alloc
 	@printf "$(YELLOW)  → Testing aimdb-core (std platform)$(NC)\n"

aimdb-data-contracts/src/linkable.rs

@@ -49,6 +49,9 @@ where
 
 #[cfg(test)]
 mod tests {
+    use alloc::string::{String, ToString};
+    use alloc::vec::Vec;
+
     use crate::{Linkable, SchemaType};
     use serde::{Deserialize, Serialize};