Enhance texture_not_found.md with full diagnostic coverage (#25)

* Initial plan

* Enhance behaviorpack/item/components/texture_not_found.md

Agent-Logs-Url: https://github.com/Blockception/Minecraft-Error-Codes/sessions/71b50b13-e4b3-47e4-90c3-1f98b5179fa7

Co-authored-by: DaanV2 <2393905+DaanV2@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: DaanV2 <2393905+DaanV2@users.noreply.github.com>

Author: Copilot

behaviorpack/item/components/texture_not_found.md

@@ -1,23 +1,91 @@
 # Item - Texture not found
 
-The item texture shortname you have specificed could not be found in the `item_texture.json` file.
+`behaviorpack.item.components.texture_not_found`
 
-For example;
+**Severity:** Error
+
+The item texture shortname you have specified could not be found in the `item_texture.json` file of any resource pack in the project, nor in the list of built-in vanilla texture shortnames.
+
+## Trigger conditions
+
+This error is raised in two situations:
+
+### 1. `minecraft:icon` on a behavior pack item
+
+The `minecraft:icon` component supports three formats, and the shortname is checked in each case:
+
+**String shorthand** — the entire value is the texture shortname:
+
+```json
+{
+  "minecraft:icon": "apple"
+}
+```
+
+**Object with `texture` property** — the `texture` field is the shortname:
 
 ```json
 {
- "minecraft:icon": "apple"
+  "minecraft:icon": {
+    "texture": "apple"
+  }
 }
 ```
 
+**Object with `textures` map (per-context textures)** — each value in the `textures` object is checked individually:
+
+```json
+{
+  "minecraft:icon": {
+    "textures": {
+      "default": "apple",
+      "charged": "apple_charged"
+    }
+  }
+}
+```
+
+### 2. Spawn egg texture on a resource pack entity
+
+The same diagnostic code is also emitted when a resource pack entity's `description.spawn_egg.texture` references a shortname that cannot be found in `item_texture.json`:
+
+```json
+{
+  "minecraft:client_entity": {
+    "description": {
+      "identifier": "my_namespace:my_entity",
+      "spawn_egg": {
+        "texture": "my_entity_spawn_egg"
+      }
+    }
+  }
+}
+```
+
+## Where the shortname is resolved
+
+The shortname is looked up in the `texture_data` object inside an `item_texture.json` file located in the resource pack:
+
 ```json
 {
   "resource_pack_name": "rp",
   "texture_name": "atlas.items",
   "texture_data": {
     "apple": {
       "textures": "textures/items/apple"
+    },
+    "apple_charged": {
+      "textures": "textures/items/apple_charged"
     }
   }
 }
 ```
+
+Vanilla built-in texture shortnames (from the Minecraft default resource pack) are also accepted and will **not** trigger this error.
+
+## How to fix
+
+1. **Check for typos** in the shortname used in `minecraft:icon` (or `spawn_egg.texture`) and correct any misspellings.
+2. **Add the missing entry** to your resource pack's `item_texture.json` under `texture_data`, mapping the shortname to a texture path.
+3. **Verify the file location** — the `item_texture.json` file must be placed at `textures/item_texture.json` inside a resource pack that is included in your project.
+4. If you intended to reuse an existing vanilla texture, **check the vanilla shortname** matches exactly (shortnames are case-sensitive).