chore: strictly ignore test_snapshots and add contributor policy

Author: bbkenny

docs/TEST_SNAPSHOTS_POLICY.md

@@ -0,0 +1,13 @@
+# Test Snapshots Policy
+
+**CRITICAL RULE FOR ALL CONTRIBUTORS:**
+
+The `test_snapshots/` directory is strictly ignored by git and **MUST NEVER** be pushed to the repository. 
+
+### Why?
+When running tests in Rust/Soroban, the framework often generates local test snapshot files (JSON). These snapshots contain local, environment-specific state or extremely verbose output that causes massive, unnecessary PR diffs (sometimes thousands of lines long).
+
+### Contributor Guidelines
+1. Do not forcefully add test snapshots using `git add -f`.
+2. Always ensure your `.gitignore` is properly ignoring `test_snapshots/`.
+3. Review your PR diffs carefully. If you see `.json` snapshot files in your PR, remove them before requesting a review.