fix(netconf-proto): handle empty containers in copy_buffer_till functions

[riccardo-negri]

A <get> reply whose filter matched nothing comes back as a
self-closing <data/> (e.g. IOS-XR when a queried dynamic subscription
is absent from the operational datastore). copy_buffer_till then found
no </data> end tag to copy up to and scanned to EOF, surfacing the
misleading SSH codec error NETCONF XML parsing error: 'Found EOF while expecting data' and failing the YANG Library fetch.

The same latent defect existed at every copy_buffer_till /
copy_buffer_till_with_namespaces call site (empty subtree filter,
<config/>, <rpc/>, <rpc-reply/>, ...): none guarded against an
empty or self-closing container. Guard both primitives with
parent_has_child() -- mirroring collect_xml_sequence* -- so all seven
call sites return empty content instead of running to EOF.

Add unit tests for the two primitives and an end-to-end rpc-reply test
covering both the <data/> and <data></data> forms.

[Copilot]

Pull request overview

This PR fixes NETCONF XML subtree copying so self-closing/empty containers (e.g. <data/> or <data></data>) no longer cause copy_buffer_till* to scan to EOF and surface misleading parsing errors, restoring correct behavior for empty <get> / <get-config> replies.

Changes:

• Add a parent_has_child() guard to copy_buffer_till and copy_buffer_till_with_namespaces to return empty content for empty/self-closing containers.
• Add unit tests covering empty/self-closing containers for both primitives.
• Add an end-to-end RpcReply test ensuring <data/> and <data></data> parse as empty data.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
crates/netconf-proto/src/xml_utils.rs	Guards the copy primitives against empty/self-closing containers and adds targeted unit tests.
crates/netconf-proto/src/protocol.rs	Adds an end-to-end RPC reply test for empty <data> forms.

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