fix: hydrate relation role-player references by IID

[ds1sqe]

Fix relation-as-role-player hydration when TypeDB fetch results include only the nested relation IID/type metadata.

These relation references are now built as partial IID-backed instances instead of going through normal constructor validation, which avoids requiring the nested relation's own role players.

Also prevent omitted optional roles from retaining class-level RoleRef defaults on relation instances.

Closes: #126

[codecov]

Codecov Report

❌ Patch coverage is 57.62712% with 25 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
type_bridge/models/relation.py	70.83%	4 Missing and 3 partials ⚠️
type_bridge/typedb_driver.py	22.22%	7 Missing ⚠️
type_bridge/models/role.py	45.45%	6 Missing ⚠️
type_bridge/crud/role_players.py	63.63%	2 Missing and 2 partials ⚠️
type_bridge/session.py	50.00%	1 Missing ⚠️

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