[cli] fix BlockwiseTransmitHook to be stateless (#12380)

This commit updates `Coap::BlockwiseTransmitHook` to calculate the
current block count based on the block position (`aPosition`) and
block length (`*aBlockLength`), rather than relying on a `static`
variable.

The previous implementation used a `static uint32_t blockCount` to
track the progress of the block-wise transfer. This approach caused
issues when multiple transfers occurred concurrently or when the
process persisted across test retries (as in the simulation
environment). In such cases, the static variable could become out of
sync, leading to incorrect transfer termination or infinite loops.

By deriving `blockCount` from `aPosition`, the hook becomes stateless
and correctly handles retransmissions and concurrent transfers.

src/cli/cli_coap.cpp

@@ -1176,8 +1176,7 @@ otError Coap::BlockwiseTransmitHook(void     *aContext,
 
 otError Coap::BlockwiseTransmitHook(uint8_t *aBlock, uint32_t aPosition, uint16_t *aBlockLength, bool *aMore)
 {
-    static uint32_t blockCount = 0;
-    OT_UNUSED_VARIABLE(aPosition);
+    uint32_t blockCount = aPosition / *aBlockLength;
 
     // Send a random payload
     otRandomNonCryptoFillBuffer(aBlock, *aBlockLength);
@@ -1189,15 +1188,13 @@ otError Coap::BlockwiseTransmitHook(uint8_t *aBlock, uint32_t aPosition, uint16_
         OutputBytesLine(&aBlock[i * 16], 16);
     }
 
-    if (blockCount == mBlockCount - 1)
+    if (blockCount >= mBlockCount - 1)
     {
-        blockCount = 0;
-        *aMore     = false;
+        *aMore = false;
     }
     else
     {
         *aMore = true;
-        blockCount++;
     }
 
     return OT_ERROR_NONE;