Reject zero-sized FIT field definitions with parse context

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I maintain Live Tennis API.

Fixes #40. Reject zero-sized ordinary and developer field definitions immediately with FitParseError, including the field number and definition offset. Such files already fail in every error mode; this makes that failure actionable and catches it even without a following data message. Positive misaligned fields still use the existing byte fallback, and definitions with no fields remain valid.

Validation on Linux:

• All 22 project tests pass on Python 3.10, 3.11, 3.12 and 3.13. The 24 zero-size regression cases fail on upstream and pass here; six compatibility cases cover existing recovery and empty definitions.
• Source archive/wheel build and twine check pass. The installed wheel handles the issue's 26-byte reproduction in all three modes with offset 14, and still decodes a valid counterpart.
• Changed production file passes Flake8. Full Flake8 has the same two existing B042 findings in fitdecode/exceptions.py on upstream and this branch.

Windows was not tested locally.