Implement 'when' statement evaluation

[steiler]

Background

YANG when statements (RFC 7950 §7.21.5) attach a conditional xpath expression to a node — the node only exists in the data tree if the expression evaluates to true. Neither the schema-server nor the data-server currently implement when evaluation.

What needs to happen

schema-server (prerequisite)

The schema-server must:

1. Serialise the when xpath string from yang.Entry.When.Name into the schema proto so data-server can retrieve it
2. Apply import-alias prefix normalisation to the xpath before serialising it — identical to the rewrite already implemented for must statements. When a module imports an identity module under a local alias different from the declared prefix, quoted literals like 'alias-id:BGP' must be rewritten to 'declared-prefix:BGP'. The normalizeXPathLiterals helper in pkg/schema/leaf.go is directly reusable for this. See Normalise import-alias prefixes in 'when' xpath expressions schema-server#240 for details on this specific requirement.

data-server

The data-server must:

1. Retrieve the normalised when xpath string from the schema proto
2. Evaluate it during validation / tree operations against the current data tree context
3. Exclude nodes whose when condition evaluates to false (node non-existence semantics per RFC 7950)

Notes

• when evaluation context follows RFC 7950 §7.21.5: the context node is the node carrying the when, the accessible tree is the full data tree
• when on a uses/augment node has slightly different context rules (§7.21.5 "context node" note) — worth scoping carefully
• The xpath evaluator already used for must should be reusable here
• Prefix normalisation in the schema-server is a hard prerequisite for correct evaluation of identity-valued comparisons in when expressions