Make server fully compatible with client.NewInProcessClient (#20)

* refactor: split filesystem related code into new handler struct

... so the actual filesystem server is just the regular `server.MCPServer` struct.
This way it can be used with the in-process client.

Also added a test to cover this functionality.

* ci(tests): add workflow for executing tests

* refactor: apply linter suggestions

* test(handler): add some test cases for the `read_file` tool

Author: jochil

.github/workflows/test.yml

@@ -0,0 +1,21 @@
+name: Test 
+
+on: [push]
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: '>=1.21.0'
+          check-latest: true
+
+      - name: Run tests
+        run: go test ./...