feat: add get_shape_library tool for AI icon discovery (#335)

* feat: add get_shape_library tool for AI icon discovery

- Add server-side tool that returns shape library documentation
- AI can fetch icon/shape names on-demand before generating diagrams
- Includes path traversal protection and input sanitization
- Library index embedded in tool description for discoverability
- Supports 33 libraries: AWS, Azure, GCP, Kubernetes, Cisco, etc.

* fix: improve get_shape_library error handling and imports

- Move fs/path imports to top of file (avoid dynamic imports per call)
- Distinguish file-not-found vs other errors in catch block
- Include invalid input in validation error message
- Log unexpected errors for debugging

* docs: add get_shape_library to system prompt tool list

- Add Tool4 (get_shape_library) to available tools section
- Add usage guidance in 'Choose the right tool' section
- Update AWS icons note to reference get_shape_library for icon discovery

* fix: display get_shape_library tool output in chat UI

* fix: correct state check for get_shape_library output display

* fix: make get_shape_library output respect fold state

* style: auto-format with Biome

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Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

docs/shape-libraries/atlassian.md

@@ -0,0 +1,32 @@
+# atlassian
+
+**Type:** SVG images
+**Path:** `img/lib/atlassian/`
+
+## Usage
+
+```xml
+<mxCell value="label" style="image;aspect=fixed;image=img/lib/atlassian/Jira_Logo.svg;" vertex="1" parent="1">
+  <mxGeometry x="0" y="0" width="60" height="60" as="geometry" />
+</mxCell>
+```
+
+## Shapes (17)
+
+- `Atlassian_Logo`
+- `Bamboo_Logo`
+- `Bitbucket_Logo`
+- `Clover_Logo`
+- `Confluence_Logo`
+- `Crowd_Logo`
+- `Crucible_Logo`
+- `Fisheye_Logo`
+- `Hipchat_Logo`
+- `Jira_Core_Logo`
+- `Jira_Logo`
+- `Jira_Service_Desk_Logo`
+- `Jira_Software_Logo`
+- `Sourcetree_Logo`
+- `Statuspage_Logo`
+- `Stride_Logo`
+- `Trello_Logo`