fix(describegpt): semanticmd review follow-ups for #3935 (stats/validation gating + tests)

[jqnatividad]

Follow-up fixes for the semanticmd Data Dictionary work that was squash-merged in #3935. These two commits were made locally after the squash-merge and weren't part of that PR, so they're landing here.

What changed

Review fixes (Copilot review of #3935):

• has_stats now considers stats.sparsity, so a column whose only retained stat is sparsity (e.g. via a custom --addl-cols-list) still renders the ### Statistics block — consistent with the Sparsity column the table renders.
• The SemanticMd template renders the length constraint gracefully when only one of min_length/max_length is present (- Length >= N / - Length <= N via minijinja elif), avoiding an empty ### Validation block.
• The committed reference doc frontmatter now matches actual yaml_scalar output: URL, spaced title/license, and the YYYY-MM-DD date are quoted; plain lat/lon are unquoted.

Regression tests (roborev follow-up):

• formatters unit tests: build_semanticmd_entry sets has_stats for a sparsity-only numeric column, and has_validation for a text column with only one of min_length/max_length.
• describegpt render test: a full semanticmd render asserting the ### Statistics block appears for a sparsity-only numeric column and the elif length branches emit - Length >= N / - Length <= N (never an empty ### Validation block).

Verification

• cargo +nightly fmt clean
• cargo test -F all_features describegpt — 77 integration + semanticmd unit tests pass (3 new)
• cargo clippy -F all_features --bin qsv clean

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[codacy-production]

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