fix: propagate application write rejections as ATT Error Responses (backport to 2.5)

[jie-meng]

Backport of #442 to the release/2.5 branch.

Summary

This backports the fix for propagating application-layer write rejections as ATT Error Responses to the 2.5 release branch.

Changes

• NimBLECharacteristic: Added setWriteError(uint8_t) and getWriteError() const methods
• NimBLECharacteristic::writeEvent(): Resets error slot before each write
• NimBLEServer::handleGattEvent(): Returns the write error for characteristic writes (descriptor writes still return 0)

Usage

Applications can now reject writes in onWrite() callbacks:

void MyCallbacks::onWrite(NimBLECharacteristic* pChar, NimBLEConnInfo& connInfo) {
    if (deviceBusy) {
        pChar->setWriteError(0x80); // vendor-specific error code
        return;
    }
    // process write normally
}

Testing

• Verified compile against ESP-IDF 5.x
• Verified the write error is correctly returned to the BLE client as ATT Error Response

@h2zero - this is the backport requested in #442 comment. Would appreciate a review/merge when you have cycles.

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[jie-meng]

@h2zero - this is the backport of #442 to release/2.5 as discussed in #442. Our downstream project is pinned to 2.5.x and needs this fix. Would appreciate a review when you have cycles.