[core] fix implicit integer conversion warnings (#11709)

This commit enables the `-Wimplicit-int-conversion` compiler flag for
`ftd`, `mtd`, and `radio` to improve code quality by detecting
potential data loss from implicit type conversions. This is enabled
when clang toolchain is used.

All resulting warnings have been addressed by either:
- Changing variable, parameter, or return types to ensure consistency
  and prevent overflows.
- Adding explicit `static_cast` where the type conversion is intended
  and safe.

tests/unit/test_routing_manager.cpp

@@ -619,7 +619,7 @@ void LogRouterAdvert(const Icmp6Packet &aPacket)
 
             VerifyOrQuit(rdnss.IsValid());
 
-            for (uint8_t index = 0; index < rdnss.GetNumAddresses(); index++)
+            for (uint16_t index = 0; index < rdnss.GetNumAddresses(); index++)
             {
                 Log("     RDNSS - %s, lifetime:%u", rdnss.GetAddressAt(index).ToString().AsCString(),
                     rdnss.GetLifetime());