fix: validate CraftEngine block ID format in fromMap at config-load time

[Copilot]

CraftEngineCustomBlock.fromMap() silently accepted blank strings and non-String values (coerced via Objects.toString) as valid block IDs, deferring failures to runtime placement.

Changes

• CraftEngineCustomBlock.fromMap() — replaces Objects.toString coercion with an instanceof String pattern match; rejects blank strings and IDs that don't satisfy namespace:key format
• isValidNamespacedKey(String) — new package-private helper; validates format without touching the CraftEngine block registry (preserving the startup timing fix from Fix false "Bad custom block" errors for CraftEngine blocks on startup #511)

// Previously accepted silently — now rejected at load time:
map.put("id", "");          // blank string
map.put("id", 42);          // non-String coerced via Objects.toString
map.put("id", "nocolon");   // missing namespace separator
map.put("id", ":key");      // empty namespace
map.put("id", "ns:");       // empty key

// Still accepted (and no registry check at load time):
map.put("id", "oneblock:common_loot_block");

• CraftEngineCustomBlockTest — added test cases for all new rejection paths and direct coverage of isValidNamespacedKey

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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