Optimize PrimitiveMapper for numeric types

[fabricio-costa]

Summary

• Optimizes PrimitiveMapper<int>, PrimitiveMapper<double>, and PrimitiveMapper<num> in mapper_container.dart to avoid unnecessary string roundtrips when the input is already a num.
• When the input is numeric, the mappers now use direct conversion (v.round(), v.toDouble(), identity) instead of num.parse(v.toString()) or double.parse(v.toString()), avoiding an intermediate string allocation.
• Falls back to string parsing for non-numeric inputs, preserving full backwards compatibility.

Before

PrimitiveMapper<int>((v) => num.parse(v.toString()).round()),
PrimitiveMapper<double>((v) => double.parse(v.toString())),
PrimitiveMapper<num>((v) => num.parse(v.toString()), num),

After

PrimitiveMapper<int>((v) => v is num ? v.round() : num.parse(v.toString()).round()),
PrimitiveMapper<double>((v) => v is num ? v.toDouble() : double.parse(v.toString())),
PrimitiveMapper<num>((v) => v is num ? v : num.parse(v.toString()), num),

Rationale

In typical deserialization from JSON (via dart:convert), numeric values are already int or double. The previous code always went through a string roundtrip (v.toString() then parse), which allocates a temporary string for every numeric field decoded. The optimized version short-circuits this with a type check, which is essentially free in Dart.

Test plan

• Existing test/primitives/primitives_test.dart passes (covers fromValue<int>('1'), fromValue<double>('1.1'), fromValue<num>(123), and error cases)
• Existing test/container/container_test.dart passes (covers fromValue<int>('2') and list decoding with mixed types like ['2', 1.5, 0])
• No other files are modified

[schultek]

Thanks, LGTM.