fix: records_from_file raises IndexError when magic number is unrecognised (closes #75)

[seam0814]

Summary

`extension_from_magic_number_in_file()` returns an empty list when the
file's content does not match any recognised magic-number signature.
`records_from_file.init` then dereferenced `compression[0]`
unconditionally:

```python
compression = extension_from_magic_number_in_file(FileName)
if compression[0] == '.xz':
...
```

So any STDF file produced by older ATE tooling that does not carry the
`.stdf` magic header (or simply the wrong header) raised

```
IndexError: list index out of range
```

at the very first line of the constructor, before the file was ever
opened — exactly the symptom in #75.

Closes #75.

Change

```diff

• compression = extension_from_magic_number_in_file(FileName)
• if compression[0] == '.xz':

• compression = extension_from_magic_number_in_file(FileName)
• compression_ext = compression[0] if compression else None
• if compression_ext == '.xz':
  ```

`compression[0]` was used at four call sites (`.xz`, `.bz2`, `.gz`,
`.zip`); each is rewritten against `compression_ext`. The empty-magic
case now falls through to the existing "assume standard binary stdf"
branch, which is already responsible for non-compressed files. No
behavioural change on any previously-handled magic value.

Test

Added `test_records_from_file_handles_unrecognised_magic_number` in
`tests/test_utils.py`. It uses `unittest.mock.patch` to force
`extension_from_magic_number_in_file` to return `[]` and then calls
`records_from_file` on a real FAR-only STDF blob, asserting the FAR
record round-trips without raising.

Full suite: 32 passed, 4 skipped, 0 failed.

Note on a latent follow-up

When the original `IndexError` fires inside `init`, `self.fd`
is never assigned, so the subsequent `del` raises a chained
`AttributeError: 'records_from_file' object has no attribute 'fd'`.
With the `IndexError` gone this path is no longer reachable from the
reported workflow, but `del` could still be made defensive in a
follow-up (`if getattr(self, 'fd', None) is not None: self.fd.close()`).
Happy to send that as a separate PR if you'd like.

Scope

Two-file change (`Semi_ATE/STDF/utils.py` + `tests/test_utils.py`),
no public API change, no behavioural change on any path that
previously worked.

[seam0814]

Gentle nudge — small guard against IndexError on unknown magic bytes in records_from_file (closes #75). Happy to rebase or rework the error path.