JP: update MAX_TEXT_LEN limits based on measured airtimes, add getMaxGroupTextLen()

Measured actual TimeOnAir via RadioLib getTimeOnAir() on RAK WisMesh Tag
(SF12/BW125, CR4/5-8) to verify ARIB STD-T108 4-second TX limit compliance.

DM limits (getMaxTextLen):
  CR4/5: 64 bytes (3874ms)
  CR4/6: 48 bytes (3874ms)
  CR4/7: 32 bytes (3678ms)
  CR4/8: 24 bytes (3547ms)

Channel message limits (getMaxGroupTextLen):
  CR4/5: 64 bytes (3710ms)
  CR4/6: 48 bytes (3678ms)
  CR4/7: 39 bytes (3907ms)
  CR4/8: 29 bytes (3809ms)

DM and channel packets differ by 1 byte overhead, warranting separate
limits for CR4/7 and CR4/8. Non-JP returns default 160 bytes unchanged.

Also apply dynamic limit to sendCommandData() and sendGroupMessage()
via getMaxTextLen()/getMaxGroupTextLen() instead of hardcoded MAX_TEXT_LEN.

Author: jirogit

src/Dispatcher.h

@@ -39,6 +39,7 @@ class Radio {
   virtual float packetScore(float snr, int packet_len) = 0;
 
   virtual int getMaxTextLen() const { return 10 * 16; }  // default: non-JP
+  virtual int getMaxGroupTextLen() const { return 10 * 16; }  // default: non-JP
 
   /**
    * \brief  starts the raw packet send. (no wait)

src/helpers/BaseChatMesh.cpp

@@ -436,7 +436,8 @@ int  BaseChatMesh::sendMessage(const ContactInfo& recipient, uint32_t timestamp,
 
 int  BaseChatMesh::sendCommandData(const ContactInfo& recipient, uint32_t timestamp, uint8_t attempt, const char* text, uint32_t& est_timeout) {
   int text_len = strlen(text);
-  if (text_len > MAX_TEXT_LEN) return MSG_SEND_FAILED;
+  int max_len = _radio->getMaxTextLen();
+  if (text_len > max_len) return MSG_SEND_FAILED;
 
   uint8_t temp[5+MAX_TEXT_LEN+1];
   memcpy(temp, &timestamp, 4);   // mostly an extra blob to help make packet_hash unique
@@ -469,7 +470,9 @@ bool BaseChatMesh::sendGroupMessage(uint32_t timestamp, mesh::GroupChannel& chan
   char *ep = strchr((char *) &temp[5], 0);
   int prefix_len = ep - (char *) &temp[5];
 
-  if (text_len + prefix_len > MAX_TEXT_LEN) text_len = MAX_TEXT_LEN - prefix_len;
+  int max_len = _radio->getMaxGroupTextLen();
+  if (text_len + prefix_len > max_len) text_len = max_len - prefix_len;
+
   memcpy(ep, text, text_len);
   ep[text_len] = 0;  // null terminator
 

src/helpers/radiolib/RadioLibWrappers.h

@@ -54,12 +54,21 @@ class RadioLibWrapper : public mesh::Radio {
   }
 
   int getMaxTextLen() const {
-    if (!isJapanMode()) return 10 * 16;  // default
+    if (!isJapanMode()) return 10 * 16;  // default 160 bytes
     uint8_t cr = getCodingRate();
-    if (cr <= 5) return 3 * 16;   // 48 bytes ~16 JP chars
-    if (cr == 6) return 2 * 16;   // 32 bytes ~10 JP chars
-    if (cr == 7) return 1 * 16 + 8; // 24 bytes ~8 JP chars
-    return 1 * 16;                 // 16 bytes ~5 JP chars
+    if (cr <= 5) return 64;  // 3874ms @ SF12/BW125/CR4-5
+    if (cr == 6) return 48;  // 3874ms @ SF12/BW125/CR4-6
+    if (cr == 7) return 32;  // 3678ms @ SF12/BW125/CR4-7
+    return 24;               // 3547ms @ SF12/BW125/CR4-8
+  }
+
+  int getMaxGroupTextLen() const {
+    if (!isJapanMode()) return 10 * 16;  // default 160 bytes
+    uint8_t cr = getCodingRate();
+    if (cr <= 5) return 64;  // 3710ms @ SF12/BW125/CR4-5
+    if (cr == 6) return 48;  // 3678ms @ SF12/BW125/CR4-6
+    if (cr == 7) return 39;  // 3907ms @ SF12/BW125/CR4-7
+    return 29;               // 3809ms @ SF12/BW125/CR4-8
   }
 
   virtual int16_t performChannelScan();