This PR fixes critical issues with XMPP stream parsing and adds a new helper function for MUC testing.
## Changes
### 1. Fix XMPP stream parsing for multiple stanzas
The XML parser was not correctly handling multiple consecutive stanzas in the stream. Fixed depth tracking to properly detect stanza boundaries.
### 2. Fix XMPP stream parsing depth tracking
Added proper depth tracking for nested XML elements to correctly identify when a complete stanza has been received.
### 3. Remove debug logging that interfered with expect tests
Removed verbose debug output that was causing expect-based functional tests to fail due to unexpected output in the stream.
### 4. Add `stbbr_for_presence_to` for MUC join presence matching
New helper function that registers a canned response for presence stanzas based on the `to` attribute. This is essential for testing MUC (Multi-User Chat) join flows where the response must be triggered by presence to a specific room JID.
**API:**
```c
void stbbr_for_presence_to(const char *to, const char *response);
Reviewed-on: jabber.developer2/stabber#1
Co-authored-by: jabber.developer2 <jabber.developer2@jabber.space>
Co-committed-by: jabber.developer2 <jabber.developer2@jabber.space>
