fix: WriteRequests initialization in FileStream.queuePWrite

[negrel]

Fix the following error and add a test for it:

test
└─ run xev-test
   └─ zig test xev-test Debug native 2 errors
src/watcher/file.zig:170:22: error: missing struct field: full_write_buffer
            req.* = .{};
                    ~^~
src/watcher/stream.zig:36:34: note: struct declared here
        pub const WriteRequest = struct {
                                 ^~~~~~