fix: map NaN/Infinity float columns to JSON null instead of failing the workflow

[Copilot]

float4/float8 columns containing NaN or ±Infinity caused the entire workflow to fail, since serde_json::Number::from_f64 returns None for non-finite values. This meant $result.nan_col-style substitutions were broken — either the column was missing or the instance was in failed state.

Changes

• src/activities/execute_sql.rs — FLOAT4/FLOAT8 decode branches now return Ok(serde_json::Value::Null) for non-finite values. After the finiteness guard, from_f64 is guaranteed to succeed, so the dead error path is replaced with .unwrap_or_else(|| panic!(...)) (value included in message). Updated module-level type table and decode_column docstring accordingly.

• tests/e2e/sql/21_typed_results.sql — Test 11 rewritten: queries NaN, Infinity, and a finite column together; asserts the instance completes, that nan_col/inf_col appear as JSON null (key present), and that the finite column remains a JSON number.

// Before: non-finite → Err → instance fails
if v.is_nan() || v.is_infinite() {
    Err(format!("FLOAT8 column '{col_name}' contains non-finite value (NaN or Inf)"))
}

// After: non-finite → null → instance continues
if v.is_nan() || v.is_infinite() {
    Ok(serde_json::Value::Null)
}