feat(esp_psram): add esp_psram_ptr_is_no_enc() helper

Drivers that allocate from the unencrypted PSRAM carve-out via
MALLOC_CAP_SPIRAM_NO_ENC currently have no way to verify after the fact
which pool a buffer came from. This is particularly relevant for callers
using heap_caps_malloc_prefer(MALLOC_CAP_SPIRAM_NO_ENC, MALLOC_CAP_SPIRAM),
where a silent fallback to encrypted PSRAM would still pass the typical
esp_ptr_external_ram() check.

Expose esp_psram_ptr_is_no_enc() in the public esp_psram.h header. It
performs a range check against the carve-out's virtual-address window and
returns false when PSRAM is not initialized or CONFIG_SPIRAM_ENC_EXEMPT is
disabled, so callers do not need to guard the call site with #if.

Also reference the helper from the External RAM documentation alongside
the heap_caps_malloc(MALLOC_CAP_SPIRAM_NO_ENC) usage example.

docs/en/api-guides/external-ram.rst

@@ -267,6 +267,8 @@ By default, failure to initialize external RAM will cause the ESP-IDF startup to
 
         ``MALLOC_CAP_SPIRAM_NO_ENC`` must be requested explicitly. It is intentionally not combined with ``MALLOC_CAP_SPIRAM`` or ``MALLOC_CAP_DEFAULT``, so ordinary SPIRAM/heap allocations cannot accidentally land in the unencrypted region.
 
+        To verify after the fact that a buffer was allocated from the unencrypted carve-out (for example after a ``heap_caps_malloc_prefer()`` call that may have fallen back to encrypted PSRAM), use :cpp:func:`esp_psram_ptr_is_no_enc`.
+
     .. only:: SOC_PSRAM_ENCRYPTION_SEPARATE_KEY
 
         On {IDF_TARGET_NAME}, PSRAM encryption can use an independent encryption key. If the PSRAM encryption key is not programmed, the flash encryption key will be used as the PSRAM encryption key.