fix(ApplyDeviate): replace Exts on deviate replace, not append

[steiler]

Overview

ApplyDeviate now copies extension statements (Exts) from deviate add and deviate replace substatements onto the deviated target Entry. Previously that propagation was missing entirely, so extensions declared inside a deviation (for example sdcio-ext:sensitive in a device-profile overlay) never appeared on the parsed leaf entry.

Motivation

RFC 7950 allows extension substatements under deviate add and deviate replace. The parser already attached them to the deviation-side Entry (devSpec.Exts), but ApplyDeviate never merged them into deviatedNode.Exts, so downstream consumers could not see them.

SDCIO relies on this for marking sensitive leaves via deviations; see related work in sdc-protos#123, schema-server#244, and data-server#452.

What changed

Inside the existing case DeviationAdd, DeviationReplace handling (alongside Default, must, etc.), add explicit Exts handling mirroring the Default pattern:

• DeviationAdd: deviatedNode.Exts = append(deviatedNode.Exts, devSpec.Exts...)
• DeviationReplace: deviatedNode.Exts = append([]*Statement{}, devSpec.Exts...) — the target node's extension list becomes exactly the deviation's list (replace semantics), not a merge that keeps unrelated prior extensions from accumulating incorrectly when the spec intends a full substitution of the deviation-supplied set.

switch dt {
case DeviationAdd:
	deviatedNode.Exts = append(deviatedNode.Exts, devSpec.Exts...)
case DeviationReplace:
	deviatedNode.Exts = append([]*Statement{}, devSpec.Exts...)
}

Tests

• deviate add propagates extensions onto target entry — extensions from deviate add reach the target leaf.
• deviate replace propagates extensions onto target entry — extensions from deviate replace reach the target leaf.
• deviate replace replaces existing extensions, does not accumulate — base leaf already has tag-ext:old-tag; overlay deviate replace supplies sdcio-ext:sensitive; result must not retain the old extension.

Test plan

• go test ./pkg/yang/... (or full module test as per repo CI)
• Confirm downstream PRs that pin this commit still build and pass sensitive-path / schema tests

[alexandernorth]

lgtm