JP_STRICT: dynamic MAX_TEXT_LEN based on runtime CR value

Instead of compile-time LORA_CR define, MAX_TEXT_LEN is now determined
at runtime by reading the actual coding rate from the radio hardware.

Added getMaxTextLen() to RadioLibWrapper and Dispatcher:
- CR4/5: 48 bytes (~16 JP chars, TX ~3808ms)
- CR4/6: 32 bytes (~10 JP chars)
- CR4/7: 24 bytes (~8 JP chars)
- CR4/8: 16 bytes (~5 JP chars, default)

getCodingRate() added to CustomSX1262Wrapper to read codingRate
from RadioLib PhysicalLayer at runtime.

Tested: 48-byte limit with LORA_CR=5, 16-byte limit with LORA_CR=8.

src/helpers/BaseChatMesh.cpp

@@ -395,8 +395,15 @@ void BaseChatMesh::onGroupDataRecv(mesh::Packet* packet, uint8_t type, const mes
 
 mesh::Packet* BaseChatMesh::composeMsgPacket(const ContactInfo& recipient, uint32_t timestamp, uint8_t attempt, const char *text, uint32_t& expected_ack) {
   int text_len = strlen(text);
+
+#ifdef JP_STRICT
+  int max_len = _radio->getMaxTextLen();
+  if (text_len > max_len) return NULL;
+  if (attempt > 3 && text_len > max_len - 2) return NULL;
+#else
   if (text_len > MAX_TEXT_LEN) return NULL;
   if (attempt > 3 && text_len > MAX_TEXT_LEN-2) return NULL;
+#endif
 
   uint8_t temp[5+MAX_TEXT_LEN+1];
   memcpy(temp, &timestamp, 4);   // mostly an extra blob to help make packet_hash unique