feat: add phase 3 bun_binary bootstrap and tests

internal/bun_binary.bzl

@@ -1,2 +1,49 @@
-def bun_binary(**_kwargs):
-    fail("bun_binary is not implemented yet")
+"""Rule for running JS/TS scripts with Bun."""
+
+
+def _bun_binary_impl(ctx):
+    toolchain = ctx.toolchains["//bun:toolchain_type"]
+    bun_bin = toolchain.bun.bun_bin
+    entry_point = ctx.file.entry_point
+
+    launcher = ctx.actions.declare_file(ctx.label.name)
+    ctx.actions.write(
+        output = launcher,
+        is_executable = True,
+        content = """#!/usr/bin/env bash
+set -euo pipefail
+
+exec \"{}\" run \"{}\" \"$@\"
+""".format(bun_bin.path, entry_point.path),
+    )
+
+    transitive_files = []
+    if ctx.attr.node_modules:
+        transitive_files.append(ctx.attr.node_modules[DefaultInfo].files)
+
+    runfiles = ctx.runfiles(
+        files = [bun_bin, entry_point] + ctx.files.data,
+        transitive_files = depset(transitive = transitive_files),
+    )
+
+    return [
+        DefaultInfo(
+            executable = launcher,
+            runfiles = runfiles,
+        ),
+    ]
+
+
+bun_binary = rule(
+    implementation = _bun_binary_impl,
+    attrs = {
+        "entry_point": attr.label(
+            mandatory = True,
+            allow_single_file = [".js", ".ts", ".jsx", ".tsx", ".mjs", ".cjs"],
+        ),
+        "node_modules": attr.label(),
+        "data": attr.label_list(allow_files = True),
+    },
+    executable = True,
+    toolchains = ["//bun:toolchain_type"],
+)