fix(core): decode legacy media in compaction checkpoints - #51409

Why

Sessions with a pre-2.0.15 native compaction containing media fail to load after upgrading: Session.MessageDecodeError. #49181 changed the embedded AI media format, but existing checkpoints still contain the old shape under version: 1.

What changes

Read legacy media at the checkpoint boundary before applying the current message schema. Both SessionProviderContext.validate() and decode() use the same normalization.

flowchart LR
    A[Stored checkpoint] --> B{Media shape}
    B -->|mediaType + data| C[Convert to media.source]
    B -->|media asset| D[Current Message decoder]
    C --> D
    D --> E[Load and replay session]

Attachment metadata is preserved. Malformed current assets still fail validation.

Scope

One Core compatibility helper; recovery happens on read without rewriting the database. Frozen legacy JSON and a database-backed history test protect this boundary.

Verification

# packages/core
bun run test test/session-provider-context.test.ts
# repository root
bun run check

The history regression persists an old-format checkpoint, reloads it, and verifies native replay and original-transcript selection. Full lint/type checks pass (35 typecheck tasks). Simplification pass completed.