Reject out-of-range JSON integer values

[sean0407]

Summary

Reader::to_basic_type now checks JSON integer values against the target type's bounds before casting. The focused regression covers the reported uint16_t overflow, signed upper/lower overflow, and valid narrow integer boundaries.

Fixes #682.

Evidence

• Before: uint16_t port parsed from {"port":100200} was accepted as 34664.
• After: The same input returns a field-specific out-of-range error. int16_t values outside both bounds are rejected; INT16_MIN and UINT16_MAX still round-trip.
• Checks: Focused tests/json/test_integers.cpp compiled with C++20 and -Wall -Werror and ran in an isolated network-disabled container (2 tests passed); git diff --check; patch policy accepted 2 text files with no binary changes.

Merge Danger

Door: Two-way

Blast Radius: Parsing

Only JSON integer conversion to narrower integral types changes; values outside the destination type's representable range now return an error.

[liuzicheng1987]

@sean0407 thanks for the contribution. I am merging.