Add resolution-strategy input to support oldest compatible version selection

Co-authored-by: eifinger <1481961+eifinger@users.noreply.github.com>

RESOLUTION_STRATEGY_DEMO.md

@@ -0,0 +1,43 @@
+# Resolution Strategy Demo
+
+This file demonstrates the new `resolution-strategy` input.
+
+## Default behavior (highest strategy)
+```yaml
+- name: Install highest compatible uv version
+  uses: astral-sh/setup-uv@v6
+  with:
+    version: ">=0.4.0"
+    # resolution-strategy: "highest" is the default
+```
+
+## Lowest strategy for testing compatibility
+```yaml
+- name: Install lowest compatible uv version  
+  uses: astral-sh/setup-uv@v6
+  with:
+    version: ">=0.4.0"
+    resolution-strategy: "lowest"
+```
+
+## Use case: Testing with matrix of strategies
+```yaml
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        resolution-strategy: ["highest", "lowest"]
+    steps:
+      - uses: actions/checkout@v5
+      - name: Install uv with ${{ matrix.resolution-strategy }} strategy
+        uses: astral-sh/setup-uv@v6
+        with:
+          version: ">=0.4.0" 
+          resolution-strategy: ${{ matrix.resolution-strategy }}
+          cache-suffix: ${{ matrix.resolution-strategy }}
+      - name: Test with strategy
+        run: |
+          echo "Testing with $(uv --version)"
+          uv run --frozen pytest
+```