fix(core): probeWritable(at:) always cleans up the probe file, even on partial failure

Two leak paths: (a) AtomicFile.write renames the temp file onto the probe
path and THEN fsyncs the containing directory — if that last fsync throws,
the probe file already exists on disk but the old code returned false
without ever attempting removal; (b) if the explicit removeItem call itself
threw, there was no retry, so a transient File Provider removal failure left
the file behind permanently.

Fix: an unconditional `defer` now checks fileExists and retries removeItem
regardless of which branch returned early. The function only reports true
when the explicit write, fsync (inside AtomicFile.write), AND removal all
succeeded AND the file is confirmed gone afterward.

New test: a FileManager subclass whose removeItem(at:) throws on its first
call (AtomicFile.write itself never touches this injected FileManager — it
uses raw Darwin/POSIX calls, not FileManager, so this only intercepts the
explicit removal + the defer's retry) asserts the function returns false AND
no probe file remains — verified this actually needs the defer by
temporarily removing it and confirming the same test then fails with a
leftover ".redline-probe-<uuid>" file (see this branch's history for the
discarded revert). The directory-fsync failure path has no injectable seam
(raw fsync(2) on an already-open fd, not parameterized by any FileManager or
other substitutable dependency, and not reproducible via chmod or other
standard test techniques) — documented in the test rather than simulated.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ZiTXPbPCSjzPVsfoweAbp

Tests/ShotdeckCoreTests/TransportSettingsTests.swift

@@ -160,6 +160,49 @@ func probeWritableFalseForAPlainFilePath() throws {
     #expect(!OneDriveLocator.probeWritable(at: filePath))
 }
 
+/// The write itself succeeds (a real probe file lands on disk via AtomicFile.write,
+/// which never touches this injected FileManager — it uses raw POSIX calls), but the
+/// FIRST call to `removeItem(at:)` throws, simulating a transient File Provider
+/// removal failure. probeWritable's own defer-based cleanup must retry and succeed
+/// (the second call through this same override falls through to `super`), so no
+/// probe file is left behind even though the function correctly still reports false
+/// (the removal it explicitly attempted did fail).
+private final class ThrowOnceOnRemoveFileManager: FileManager, @unchecked Sendable {
+    private let lock = NSLock()
+    private var hasThrown = false
+
+    override func removeItem(at URL: URL) throws {
+        lock.lock()
+        let shouldThrow = !hasThrown
+        hasThrown = true
+        lock.unlock()
+        if shouldThrow {
+            throw NSError(domain: "ShotdeckCoreTests.ThrowOnceOnRemove", code: 1)
+        }
+        try super.removeItem(at: URL)
+    }
+}
+
+@Test
+func probeWritableFalseAndLeavesNoProbeFileWhenRemoveItemThrowsOnce() throws {
+    // Directory fsync failure (the OTHER way probeWritable's cleanup can be needed) has
+    // no injectable seam: AtomicFile.write's directory fsync is a raw Darwin fsync(2)
+    // call on an already-open file descriptor, not parameterized by any FileManager or
+    // other dependency this test can substitute, and there is no portable way to make
+    // fsync(2) itself fail via chmod or other standard test techniques (fsync failures
+    // are OS/filesystem/hardware-level events). Covering the removeItem-throws path
+    // (below) is what this test does; the fsync-throws path is covered by code
+    // inspection only — the same `defer` block guards both.
+    let dir = try makeTransportTemporaryDirectory(prefix: "shotdeck-probe-remove-throws")
+    defer { try? FileManager.default.removeItem(at: dir) }
+    let injectedFileManager = ThrowOnceOnRemoveFileManager()
+
+    #expect(!OneDriveLocator.probeWritable(at: dir, fileManager: injectedFileManager))
+
+    let leftovers = try FileManager.default.contentsOfDirectory(atPath: dir.path)
+    #expect(leftovers.isEmpty)
+}
+
 struct TransportDefaultsSuite {
     let name: String
     let defaults: UserDefaults