fix: use the computed size of the Offsets struct instead of hard-co… (#25816)

### What does this PR do?

- I was trying to understand how the SEA bundling worked in Bun and
noticed the size of the `Offsets` struct is hard-coded here to 32. It
should use the computed size to be future proof to changes in the
schema.

### How did you verify your code works?

- I didn't. Can add tests if this is not covered by existing tests.
ChatGPT agreed with me though. =)

src/StandaloneModuleGraph.zig

@@ -1213,7 +1213,7 @@ pub const StandaloneModuleGraph = struct {
             }
         }
 
-        if (read_amount < trailer.len + @sizeOf(usize) + 32)
+        if (read_amount < trailer.len + @sizeOf(usize) + @sizeOf(Offsets))
             // definitely missing data
             return null;