ffi: validate the value passed to the float setters - #65342

Problem

Every other setter in SetValue<T>() requires IsNumber() before it writes, so setInt8(ptr, 0, '5') throws. The floating-point branch calls ToNumber() instead, so setFloat64(ptr, 0, '1.5') writes 1.5, {} writes NaN and null writes 0. The same double is type-checked on the argument path, where ToFFIArgument() throws Argument %s must be a double, and the documentation already says the setters "validate the supplied JavaScript value against the target native type before writing it into memory".

ToNumber() also discards a pending exception. When the value has a valueOf() that throws, the branch calls THROW_ERR_INVALID_ARG_VALUE on top of the exception V8 has already scheduled, so the caller sees Value must be a number instead of their own error. DataView.prototype.setFloat64() and Buffer.prototype.writeDoubleLE() both propagate it, and src/README.md asks for an early return instead.

Fix

Check IsNumber() instead. It runs before any conversion, so valueOf() is never invoked and there is no pending exception left to discard. The messages follow the integer setters and ToFFIArgument() rather than the old generic Value must be a number.

Compatibility

NaN and Infinity still pass, since both are number values; only the type check is new. This makes input that is accepted today throw, but node:ffi is experimental and the recent fixes in this area went the same way without a semver-major label.

Tests

Six assertions are added to the setter block in test/ffi/test-ffi-memory.js, which covered only the integer setters. The last one passes { valueOf: common.mustNotCall() }, so the test fails if the setter ever converts the value again.

Fixes: #65341
Refs: #62858
Refs: #64614
Refs: #64691
Refs: #65032