fix(puffin): add missing magic number validation for puffins

[dentiny]

What changes are included in this PR?

We have two functions to read puffin metadata: read and read_with_prefetch. In theory these two functions should be similarly the same apart from IO-related logic, but read_with_prefetch seems to miss magic number validation:

iceberg-rust/crates/iceberg/src/puffin/metadata.rs

Line 284 in 4f21e50

FileMetadata::check_magic(&first_four_bytes)?;

In terms of implementation, I thought about extracting validation+footer parse into a separate function, but the code is very simple here, don't think it necessary.

Are these changes tested?

Yes, the newly added regression test fails for the current codebase

[xanderbailey]

Thanks for the fix @dentiny. Left a comment on the test, let me know what you think.

[xanderbailey]

@CTTY this PR makes sense to me, would you be able to take a look when you have a minute?

[CTTY]

LGTM!

Interestingly, java's PuffinReader doesn't validate the MAGIC at the start of file, it only validates the start and the end of puffin footer: https://github.com/apache/iceberg/blob/main/core/src/main/java/org/apache/iceberg/puffin/PuffinReader.java#L68