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/rack.md

@@ -0,0 +1,57 @@
+# rack
+
+**Type:** mxgraph shapes
+**Prefix:** `mxgraph.rack`
+
+## Usage
+
+```xml
+<mxCell value="label" style="shape=mxgraph.rack.f5.arx_500;strokeColor=#666666;" vertex="1" parent="1">
+  <mxGeometry x="0" y="0" width="200" height="30" as="geometry" />
+</mxCell>
+```
+
+Shapes are organized by vendor: `mxgraph.rack.{vendor}.{model}`
+
+## Vendors
+
+### F5
+
+- `arx_500`
+- `big_ip_1600`
+- `big_ip_2000`
+- `big_ip_4000`
+
+### Dell
+
+- `dell_poweredge_1u`
+- `poweredge_630`
+- `poweredge_730`
+
+### HPE Aruba
+
+HPE Aruba shapes have subcategories: `mxgraph.rack.hpe_aruba.{category}.{model}`
+
+**gateways_controllers:**
+- `aruba_7010_mobility_controller_front`
+- `aruba_7010_mobility_controller_rear`
+- `aruba_7024_mobility_controller_front`
+- `aruba_7205_mobility_controller_front`
+
+**security:**
+- `aruba_clearpass_c1000_front`
+- `aruba_clearpass_c2000_front`
+- `aruba_clearpass_c3000_front`
+
+**switches:**
+- `j9772a_2530_48g_poeplus_switch`
+- `j9773a_2530_24g_poeplus_switch`
+- `jl253a_aruba_2930f_24g_4sfpplus_switch`
+
+### General (rackGeneral)
+
+Use `mxgraph.rackGeneral.{shape}` for generic rack items:
+- `rackCabinet3`
+- `plate`
+
+(See draw.io Rack shape library for complete list)