feat(buzz-agent): persist the OAuth token cache owner-only on Windows

On Windows the token cache was memory-only because std has no way to set
an owner-only DACL, and the Win32 calls need `unsafe`, which buzz-agent
forbids. Each process therefore ran its own sign-in, and Databricks
stayed gated off on Windows in the desktop app.

The new buzz-private-file crate holds that FFI behind a safe API, so
buzz-agent keeps `forbid(unsafe_code)` on every platform:

- `create_new` creates a file with the protected DACL `D:P(A;;FA;;;OW)`
  set atomically at creation, the Windows counterpart of `mode(0o600)`.
- `open_owned` opens without following a final link, refuses anything
  but a regular file owned by the process's default owner, and then
  re-applies that DACL on the open handle, like the Unix `O_NOFOLLOW` +
  `fchmod` repair.

auth.rs now persists and reads the cache on Windows through these, with
the same temp-file + rename lifecycle as Unix. Tests that were Unix-only
only because Windows had no disk cache now run on both, and Windows CI
runs the helper crate (including its elevated planted-owner and symlink
tests) plus the buzz-agent token-cache unit tests.

Signed-off-by: Pinky <5f5ab050ec58ae208332edd544ebf705221e24c1b86d82a6ca07038a7a8f6ac9@buzz.block.builderlab.xyz>

.github/workflows/_ci-rust.yml

@@ -154,20 +154,23 @@ jobs:
       - name: Test (ACP launch-prefix platform contract)
         # Execute the non-Unix refusal branch; a cross-compile cannot test it.
         run: cargo test -p buzz-acp --target $env:TARGET --lib acp::launch::platform_tests
+      - name: Test (owner-only private files)
+        # The Win32 DACL code behind buzz-agent's token cache. The ignored
+        # tests plant a foreign-owned file and a symlink, which needs the
+        # runner's elevated shell.
+        run: cargo test -p buzz-private-file --target $env:TARGET -- --include-ignored
       - name: Test (buzz-agent auth coordinator)
         # The auth coordinator single-flights on an OS advisory lock, which is
-        # LockFileEx on Windows; this integration suite drives real second
-        # processes on the same lock file, so it only exercises the Windows
-        # lock runtime if it runs ON Windows. Every other job compiles it but
-        # never executes it. Tests exercised on Windows: lock serialization
-        # (two coordinators race for the same key), cooldown sidecar sharing
-        # across processes, attempt-sidecar adoption (UserInitiated waiter
-        # adopts a predecessor's denial), and the in-process single-flight for
-        # same-key coalescing. Tests that are UNIX-ONLY and NOT executed here:
-        # crash-release (flock drop on SIGKILL, guarded by #[cfg(unix)]) and
-        # cross-process cache success/race (on-disk token handoff, also
-        # #[cfg(unix)]).
+        # LockFileEx on Windows, and hands tokens across processes through the
+        # owner-only disk cache. This integration suite drives real second
+        # processes on the same lock and cache files, so it only exercises the
+        # Windows runtime if it runs ON Windows. Every other job compiles it but
+        # never executes it.
         run: cargo test -p buzz-agent --target $env:TARGET --test databricks_auth_coordinator
+      - name: Test (buzz-agent token cache)
+        # Disk cache reuse, joiner recovery and concurrent savers through the
+        # Windows owner-only cache path.
+        run: cargo test -p buzz-agent --target $env:TARGET --lib -- auth::tests databricks::tests
       # Smoke-test the new host-prereq contract: Git for Windows (which provides
       # bash) is available on the runner, a shell command round-trips, and bash
       # does NOT resolve from System32 (so WSL's launcher is never picked up).