fix: widen Edm.Decimal facets that leave no integral digits

[jrosskopf]

Fixes #254.

The defect

A TOPdesk reporting service publishes every money column as Edm.Decimal with a declared Scale and no Precision. An absent Precision is unbounded in CSDL, but BuildDecimalLogicalType defaulted it to 18, so Scale="18" produced DECIMAL(18,18) — eighteen fractional digits with no room for a single integral one. Every value of 1 or more then failed to cast and came back NULL:

erpl_web warning: 2 values in 1 column could not be converted and were returned as NULL (odata_read).
  column 'serviceagreementcost': 2 failures; first offending value: 490.0; first error:
  Could not cast value 490.000000 to DECIMAL(18,18)

The same degenerate type also came out of a declared Precision <= Scale, because the scale was truncated down to the precision rather than the precision widened. Both shapes are fixed, so the fix does not depend on which one the service emits.

The rule

CSDL facets	before	after
Scale absent or "variable"	DOUBLE	DOUBLE (unchanged, #80)
Scale="18", no Precision	DECIMAL(18,18)	DECIMAL(38,18)
Precision="18" Scale="18"	DECIMAL(18,18)	DECIMAL(38,18)
Precision="5" Scale="10"	DECIMAL(5,5)	DECIMAL(38,10)
Precision="13" Scale="3"	DECIMAL(13,3)	unchanged
Precision="47" Scale="2"	DECIMAL(38,2)	unchanged
Precision="19" Scale="0"	DECIMAL(19,0)	unchanged
Scale >= 38	DECIMAL(38,38)	DOUBLE

No mapping that works today is narrowed; only the degenerate ones widen, and only a scale too wide for any DuckDB DECIMAL falls back to DOUBLE.

Tests

• test/cpp/test_odata_edm.cpp — the facet matrix above, including a tinyxml2 parse of the reported CSDL shape.
• test/cpp/test_odata_server_e2e.cpp — an end-to-end case against ODataTestServer: real metadata parsing, real catalog, real JSON conversion, only the remote service stubbed. It reproduces the report through both ATTACH ... (TYPE odata) (the reporter's own SELECT DISTINCT) and odata_read(), and before the fix it failed with the exact warning quoted above.

erpl_web_tests: 3146 assertions in 582 cases, all green. SQLLogicTests: all green.

Docs are updated in a companion PR on the landing page (Type Mapping + a Troubleshooting entry).

🤖 Generated with Claude Code

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