*: Add presubmit script

Adds a `presubmit.sh` script to the repository root to automate
CI/CD checks. The script ensures code quality by running a series
of checks in the `rust/` directory.

Using `set -eux`, the script is configured to exit immediately if
any of the following steps fail:
 - Build the project with `cargo build`
 - Check code formatting with `cargo fmt -- --check`
 - Run the test suite with `cargo test`

Bug: b/439013538
Test: Ran the script locally
Change-Id: I0344c0e494bda49e0b09778f5cfdd0d3e4c01f26
Reviewed-on: https://bluetooth-review.googlesource.com/c/bluetooth/+/2700
Reviewed-by: Rob Mohr <mohrr@google.com>
Reviewed-by: Marie Janssen <jamuraa@google.com>

presubmit.sh

diff --git a/presubmit.sh b/presubmit.sh
new file mode 100755
index 0000000..3da037b
--- /dev/null
+++ b/presubmit.sh
@@ -0,0 +1,24 @@
+#!/bin/bash
+# Presubmit script for CI/CD
+
+# Exit on error, print commands, and exit on unset variables.
+set -eux
+
+# Change to the rust directory to run cargo commands.
+cd "$(dirname "$0")/rust"
+
+# 1. Build the code.
+# `set -e` will cause the script to exit if this fails.
+cargo build
+
+# 2. Check formatting.
+# `cargo fmt -- --check` will exit with a non-zero status code if any
+# files are not formatted correctly.
+cargo fmt -- --check
+
+# 3. Run tests.
+# `set -e` will cause the script to exit if any tests fail.
+cargo test
+
+{ set +x; } 2>/dev/null
+echo "Presubmit checks passed!";