fix(pkgmetadata): raise ValueError when no classifier maps to SPDX

[rd4398]

_trove_to_spdx() skipped every classifier absent from _TROVE_SPDX, leaving trove_spdx empty and calling _parse_spdx(""), which parses to None and raised AttributeError on None.simplify().

Since that is not a ValueError, license_from_metadata_values() could not catch it and fall back. It now raises ValueError when no classifier maps to SPDX, restoring the documented behavior so callers can fall back to NOASSERTION.

Closes: #1372

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Repository: python-wheel-build/fromager/.coderabbit.yaml
• Review profile: CHILL
• Plan: Advanced
• Run ID: cb75e581-c3a5-49a8-99da-05ce9ad7ab2a

📥 Commits

Reviewing files that changed from the base of the PR and between f699f2f and 81056e3.

📒 Files selected for processing: 2

• src/fromager/pkgmetadata/pep639.py
• tests/test_pkgmetadata.py

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📝 Walkthrough

When no classifier maps to an SPDX expression, _trove_to_spdx now raises ValueError instead of passing an empty expression to the parser. Tests add unmapped classifier cases and a case combining a Python-version classifier with an ambiguous BSD license classifier.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 81056

The change reports unmapped classifier metadata as a license-detection error while preserving tested mapped results. No actionable merge risk remains.

Pre-merge checks: 4

Check name	Status	Explanation
Description check		The description clearly explains the empty SPDX expression failure, the resulting exception, and the ValueError fix. It matches the changeset and PR objective.
Title check		The title clearly and concisely describes the main change: raising ValueError when no classifier maps to an SPDX expression.
Linked Issues check		Issue #1372 requires a ValueError-compatible failure when metadata contains no usable license. At the reviewed head, _trove_to_spdx() raises `ValueError("no classifier maps to a SPDX license express…
Out of Scope Changes check		The reviewed changes are limited to the classifier error handling in src/fromager/pkgmetadata/pep639.py and regression cases in tests/test_pkgmetadata.py. Both changes directly support issue #1372…

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[mprpic]

LGTM!