Keep a DbType that was set explicitly when Value is assigned afterwards

[alaricd]

DuckDBParameter.Value replaced DbType with the type inferred from the new value on every assignment, so an explicit DbType followed by a Value assignment was lost:

var p = new DuckDBParameter { DbType = DbType.Time };
p.Value = new TimeSpan(13, 45, 30);
// p.DbType is no longer DbType.Time

Why it matters. #360 binds a TimeSpan with no inferred type as an INTERVAL, except when DbType.Time says it is a time of day. Because the Time was lost, a TimeSpan for a TIME column inside a MERGE source (where DuckDB has no target column to infer from) is bound as an INTERVAL, and DuckDB cannot cast that to TIME:

MERGE INTO t USING (SELECT $1 AS id, $2 AS v) s ON t.id = s.id
WHEN MATCHED THEN UPDATE SET v = s.v
WHEN NOT MATCHED THEN INSERT (id, v) VALUES (s.id, s.v)
-- Conversion Error: Unimplemented type for cast (INTERVAL -> TIME) when casting from source column v

It worked with 1.5.6 (the value was sent as text) and fails with 1.5.6.1. The same applies to any explicit DbType set before Value, which is the usual order for a provider-neutral caller (CreateParameter(), set DbType, set Value).

Fix. A DbType that was assigned now stays. Until one is assigned, and after ResetDbType(), the type still follows the value, as it does for SqlParameter and NpgsqlParameter. The constructors that take a value without a type still infer it; the ones that take a DbType keep it.

Tests (Parameters/ParameterDbTypeTests): DbType set before and after the value is kept; inference when none was set and after ResetDbType(); the constructors; the MERGE case above, which fails without the change; and a bare TimeSpan is still bound as an interval. The new tests pass 5 of 5 runs, and the suite (everything except the VARINT and connection stress tests) passes: 1285 tests on net8.0.

Found while testing for a release of https://github.com/pengdows/pengdows.crud

[coveralls]

Coverage Report for CI Build 39136404700

Coverage decreased (-0.03%) to 92.52%

Details

• Coverage decreased (-0.03%) from the base build.
• Patch coverage: 19 of 19 lines across 1 file are fully covered (100%).
• 2 coverage regressions across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

2 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
DuckDB.NET.Data/Connection/ConnectionManager.cs	2	82.83%

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Coverage Stats

Relevant Lines:	4023
Covered Lines:	3786
Line Coverage:	94.11%
Relevant Branches:	2287
Covered Branches:	2052
Branch Coverage:	89.72%
Branches in Coverage %:	Yes
Coverage Strength:	997737.26 hits per line

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