refactor(framework): Add FFS-replacement methods to LinkState/NodeState

[danieljanes]

This PR:

• Adds two methods to LinkState and NodeState (store_fab and get_fab)
• Implements those methods in all implementations
• Add DB migrations

[Copilot]

Pull request overview

This PR adds first-class FAB (Flower App Bundle) storage and retrieval APIs to the state layer (LinkState/NodeState) as a replacement for the previous FFS-style handling, including persistence via a new SQL table and migration.

Changes:

• Introduces store_fab/get_fab abstract methods on LinkState and NodeState and implements them for in-memory and SQL-backed state.
• Adds a new fab table to the LinkState SQL schema plus an Alembic migration to create it.
• Extends existing state test suites to cover FAB storage, deduplication-by-hash, and missing-hash behavior.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
framework/py/flwr/supernode/nodestate/nodestate_test.py	Adds NodeState tests for storing/retrieving FABs, deduplication by content hash, and missing lookups.
framework/py/flwr/supernode/nodestate/nodestate.py	Extends the NodeState interface with store_fab/get_fab.
framework/py/flwr/supernode/nodestate/in_memory_nodestate.py	Implements FAB storage in the in-memory NodeState backend keyed by SHA-256(content).
framework/py/flwr/supercore/state/schema/linkstate_tables.py	Adds the fab table to LinkState SQLAlchemy metadata.
framework/py/flwr/supercore/state/alembic/versions/rev_2026_03_21_add_fab_table.py	Introduces an Alembic migration to create/drop the fab table.
framework/py/flwr/supercore/state/alembic/utils_test.py	Adds a migration test asserting the fab table exists after running migrations.
framework/py/flwr/server/superlink/linkstate/sql_linkstate.py	Implements FAB persistence in SqlLinkState (upsert + retrieval).
framework/py/flwr/server/superlink/linkstate/linkstate_test.py	Adds LinkState tests for FAB store/get, deduplication, and missing hash behavior.
framework/py/flwr/server/superlink/linkstate/linkstate.py	Extends the LinkState interface with store_fab/get_fab.
framework/py/flwr/server/superlink/linkstate/in_memory_linkstate.py	Implements FAB storage in the in-memory LinkState backend keyed by SHA-256(content).

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