[otns] make various enhancements to Otns (#11643)

This commit contains various enhancements to the `Otns` class:

- Makes `Otns` methods non-static. This aligns their use with the
  clang-tidy `readability-static-accessed-through-instance` check,
  which disallows accessing static methods through an instance.
- Updates the `make-pretty` script to enable OTNS and include it in
  `clang-tidy` checks.
- Adds a stub implementation of `otPlatOtnsStatus()` in the simulation
  and fake platforms. This allows the OTNS feature to be enabled in
  `make-pretty` builds and covered by GitHub Action CI checks.
- Simplifies the `EmitStatus` methods to use the `String` class for
  constructing the status string.
- Adds a new helper method to construct the CoAP status string,
  removing duplicated code.

examples/platforms/simulation/misc.c

@@ -122,3 +122,7 @@ void otPlatAssertFail(const char *aFilename, int aLineNumber)
     assert(false);
     exit(1);
 }
+
+#if OPENTHREAD_CONFIG_OTNS_ENABLE && !OPENTHREAD_SIMULATION_VIRTUAL_TIME
+void otPlatOtnsStatus(const char *aStatus) { OT_UNUSED_VARIABLE(aStatus); }
+#endif

examples/platforms/simulation/platform-config.h

@@ -76,17 +76,6 @@
 #define OPENTHREAD_CONFIG_NCP_SPI_ENABLE 0
 #endif
 
-/**
- * Check OTNS configurations
- */
-#if OPENTHREAD_CONFIG_OTNS_ENABLE
-
-#if !OPENTHREAD_SIMULATION_VIRTUAL_TIME
-#error "OTNS requires virtual time simulations"
-#endif
-
-#endif // OPENTHREAD_CONFIG_OTNS_ENABLE
-
 /**
  * @def OPENTHREAD_SIMULATION_MAX_NETWORK_SIZE
  *