fix(tags): raise InvalidTag for malformed tag field counts

[r266-tech]

Summary

parse_tag() already raises InvalidTag for empty tag components, but malformed tags with too few or too many hyphen-separated fields still reached the tuple unpack and leaked ValueError.

This adds an explicit field-count check before unpacking so inputs such as py3-none and py3-none-any-extra follow the same public error contract.

Verification

• uv run pytest tests/test_tags.py -q
• uv run ruff check src/packaging/tags.py tests/test_tags.py
• uv run ruff format --check src/packaging/tags.py tests/test_tags.py
• Codex adversarial review: VERDICT: approve

[brettcannon]

EAFP over LBYL:

Suggested change

	try:
	interpreters, abis, platforms = component_parts
	except ValueError as exc:
	raise InvalidTag(f"Tag {tag!r} must have exactly three components") from exc