Support Validation of union types

[steiler]

This PR fixes validation gaps for YANG leaves defined as union types.

Today, union branch selection happens at import time (XML/JSON/proto): the server tries branches in order and stores the first successfully parsed TypedValue. Later, validation runs against the outer union schema type instead of the specific matched branch type. Because the outer union type does not carry branch-specific constraints, several validators can no-op unexpectedly.

Problem
For union-typed leaves, the current validation path can miss real violations:

Pattern and length checks read constraints from the outer union type, which has none.
Range checks read constraints from the outer union type, which has none.
Leafref checks look for a leafref path on the outer union type, which is empty.
This means values that actually matched a leafref branch can bypass live-tree reference validation, allowing dangling references to pass undetected.

[codecov]

Codecov Report

❌ Patch coverage is 82.66667% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...kg/tree/ops/validation/validation_entry_leafref.go	70.00%	4 Missing and 2 partials ⚠️
pkg/tree/types/update.go	72.22%	5 Missing ⚠️
pkg/tree/ops/validation/validation_entry_length.go	87.50%	1 Missing ⚠️
...kg/tree/ops/validation/validation_entry_pattern.go	87.50%	1 Missing ⚠️

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