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/bpmn.md

@@ -0,0 +1,60 @@
+# bpmn
+
+**Type:** mxgraph shapes
+**Prefix:** `mxgraph.bpmn`
+
+## Usage
+
+```xml
+<mxCell value="label" style="shape=mxgraph.bpmn.shape;symbol=message;outline=throwing;" vertex="1" parent="1">
+  <mxGeometry x="0" y="0" width="60" height="60" as="geometry" />
+</mxCell>
+```
+
+## Parameters
+
+- `outline` - Event type: `start`, `end`, `catching`, `throwing`, `none`
+- `symbol` - Icon inside: `message`, `timer`, `error`, `cancel`, `compensation`, `link`, `terminate`, `general`, `multiple`, `rule`
+
+## Shapes (40)
+
+- `ad_hoc`
+- `business_rule_task`
+- `cancel_end`
+- `cancel_intermediate`
+- `compensation`
+- `compensation_end`
+- `compensation_intermediate`
+- `error_end`
+- `error_intermediate`
+- `gateway`
+- `gateway_and`
+- `gateway_complex`
+- `gateway_or`
+- `gateway_xor_(data)`
+- `gateway_xor_(event)`
+- `general_end`
+- `general_intermediate`
+- `general_start`
+- `link_end`
+- `link_intermediate`
+- `link_start`
+- `loop`
+- `loop_marker`
+- `manual_task`
+- `message_end`
+- `message_intermediate`
+- `message_start`
+- `multiple_end`
+- `multiple_instances`
+- `multiple_intermediate`
+- `multiple_start`
+- `mxgraph.bpmn`
+- `rule_intermediate`
+- `rule_start`
+- `script_task`
+- `service_task`
+- `terminate`
+- `timer_intermediate`
+- `timer_start`
+- `user_task`