fix: include filename in UnicodeDecodeError during vault gather (#32)

[MohammadYusif]

Summary

• When a vault note contains non-UTF-8 bytes, reading it raised a bare error with no indication of which file was at fault — unworkable for a user with tens of thousands of notes (see issue).
• Worse, the UnicodeDecodeError from f.read() in _get_md_front_matter_and_content was being swallowed by the trailing bare except:, which then tried to return {}, file_string where file_string was never assigned — so the user actually saw a confusing UnboundLocalError instead of the real decode error.
• This fix catches UnicodeDecodeError explicitly and re-raises it with the offending filepath appended to the message, so the failing file is immediately identifiable.

Changes

• obsidiantools/md_utils.py: in _get_md_front_matter_and_content, added an explicit except UnicodeDecodeError (before the catch-all) that re-raises a UnicodeDecodeError with (in file: <filepath>) appended to the reason. Placed ahead of the bare except: so the decode error is no longer swallowed.
• tests/test_md_utils.py: added test_non_utf8_file_error_includes_filepath, which writes a temp file with an invalid 0xff byte, calls the public get_front_matter, and asserts the raised UnicodeDecodeError message contains the filepath. Verified this test fails on the unpatched code (it raised UnboundLocalError) and passes with the fix.

Fixes #32