feat: Add OpenTelemetry tracing to @instrument_tool decorator

Enhances the @instrument_tool decorator to create distributed traces
for all MCP tool executions, improving observability and debugging.

Changes:
- Modified @instrument_tool to wrap tool execution in trace_operation
- Added automatic span creation with mcp.tool.* span names
- Sanitized tool arguments before adding to span attributes
  (excludes password, token, secret, api_key, etag, ctx)
- Limited argument strings to 500 characters to prevent huge spans
- Maintained existing Prometheus metrics functionality
- Updated docs/observability.md to reflect correct decorator name
- Added comprehensive unit tests

All ~50+ MCP tools now emit traces automatically without code changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/observability.md

@@ -243,7 +243,7 @@ If you see cardinality warnings:
 The observability stack integrates at multiple layers:
 
 1. **HTTP Layer**: `ObservabilityMiddleware` tracks all HTTP requests
-2. **MCP Layer**: Tools use `@trace_mcp_tool` for span creation
+2. **MCP Layer**: Tools use `@instrument_tool` for automatic metrics and trace span creation
 3. **Client Layer**: `BaseNextcloudClient` tracks all API calls
 4. **OAuth Layer**: Token operations are traced and metered
 5. **Background Tasks**: Vector sync operations emit metrics/traces