schemagen: accept integral exponent/decimal numbers for integer slots

JSON Schema's 'integer' type matches any number with a zero fractional
part, but the generated builder rejected forms like 1e3 or 2.0 because it
only ran std::from_chars<int64_t>. Keep that exact path for plain integer
literals (so large values near INT64_MAX stay exact), and fall back to
parsing as double for the rest, requiring an integral value within int64
range. Add <cmath> for std::trunc and cover the new cases in test_gen.

contrib/schemagen/weaseljson_schemagen.py

@@ -487,6 +487,7 @@ class Emitter:
 #pragma once
 
 #include <charconv>
+#include <cmath>
 #include <cstdint>
 #include <map>
 #include <memory>
@@ -928,8 +929,21 @@ private:
     if (si.cat == Cat::Int) {{
       int64_t v = 0;
       auto r = std::from_chars(b, e, v);
-      if (r.ec != std::errc() || r.ptr != e) {{ reject(); return; }}
-      *(int64_t *)p = v;
+      if (r.ec == std::errc() && r.ptr == e) {{
+        *(int64_t *)p = v;  // plain integer literal: parsed exactly
+      }} else {{
+        // JSON Schema "integer" accepts any number with no fractional part,
+        // including exponent/decimal forms like 1e3 or 2.0. Parse those as a
+        // double and require an integral value within int64 range.
+        double d = 0;
+        auto rd = std::from_chars(b, e, d);
+        if (rd.ec != std::errc() || rd.ptr != e || d != std::trunc(d) ||
+            d < -9223372036854775808.0 || d >= 9223372036854775808.0) {{
+          reject();
+          return;
+        }}
+        *(int64_t *)p = (int64_t)d;
+      }}
     }} else {{
       double v = 0;
       auto r = std::from_chars(b, e, v);