fix(bindings): fix SpanSet serialization size and floatspan distance datum

[estebanzimanyi]

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Summary

• Union_* and Minus_* span functions used sizeof(*ret) (fixed-size header only) when copying SpanSet results into DuckDB blob storage — multi-element SpanSets (e.g. [1,6]-[3,4] = {[1,3),(4,6]}) silently lost trailing elements, causing corrupted output or segfaults. Fixed by using spanset_mem_size(ret) with direct StringVector::AddStringOrBlob.
• minus_span_span returns NULL (not empty SpanSet) when inputs are equal — test expectation corrected from {} to NULL.
• Distance_span_value / Distance_value_span T_FLOATSPAN case was missing DatumGetFloat8() around the MEOS return value, causing the raw int64_t bit pattern (~4.6e18) to be stored instead of the actual double. Fixed both call sites; Distance_span_span was already correct.
• Parity test 005_span_ops: unskipped the 3 union queries now that union works; corrected expected values to SpanSet format {[1,3]} / {[1,5]}.

Test plan

• 005_span_ops.test passes fully (union + minus + distance sections)
• Multi-element SpanSet difference {[1,3),(4,6]} round-trips correctly
• Float distance 1.0 <-> floatspan '[2,3]' returns 1 not 4.6e18

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[estebanzimanyi]

Consolidation note — overlaps with #107

Attempted to consolidate this PR with #107 fix(bindings): fix distance return types; add + shift alias for tstzset/tstzspan+interval (both span correctness fixes — qualify as a thematically-related batch per docs/PR-COORDINATION.md on #120).

The cherry-pick hit substantive conflicts in src/temporal/span_functions.cpp:

• This PR introduces SecondsToInterval(...) helper; fix(bindings): fix distance return types; add + shift alias for tstzset/tstzspan+interval #107 introduces SecsToInterval(...) in src/include/time_util.hpp. Same operation, different name.
• This PR's Distance_span_span simplifies to a 2-branch (TSTZSPAN vs other) impl; fix(bindings): fix distance return types; add + shift alias for tstzset/tstzspan+interval #107 expands it to a full 4-case switch with per-span-type return types (INTEGER for intspan, BIGINT for bigintspan, DOUBLE for floatspan, INTERVAL for tstzspan).

Per the policy I won't blind-resolve — the maintainer's judgment is needed on:

1. Which helper name to keep (SecondsToInterval or SecsToInterval).
2. Whether to take the simpler (this PR) or the per-type-correct (PR fix(bindings): fix distance return types; add + shift alias for tstzset/tstzspan+interval #107) Distance_span_span shape.

Suggested resolution: either close this PR in favour of #107 (which is the broader correctness fix) and confirm #107 covers the SpanSet serialization size fix, or rebase #107 on top of this one and reconcile the two. Posting here rather than guessing.