tikv: support CES keyspace-level transaction safepoint

[pingyu]

What problem does this PR solve?

Essential v1 keyspaces advertise keyspace-level GC with safe_point_version=v2, while client-go only recognizes the native gc_management_type=keyspace_level metadata. The compatibility transaction safepoint loader therefore reads the unified GC key instead of the keyspace-scoped key.

What is changed and how it works?

• Route both native and Essential v1 keyspace-level GC metadata to /keyspaces/tidb/{id}/tidb/store/gcworker/saved_safe_point.
• Keep all other keyspaces on the unified GC key.
• Add focused coverage for nil, native, Essential v1, and unified metadata.

Stack dependencies

• Draft and stacked on *: update kvproto to apiv3 #2024 for API-v3 keyspace metadata compatibility.
• Uses the PD predicate introduced by client: recognize v2 keyspace-level GC pd#11100.

Tests

• go test ./tikv -run ^TestCompatibleTxnSafePointPath$ -count=1 -v
• gopls check tikv/compatible_txn_safe_point_loader.go tikv/compatible_txn_safe_point_loader_test.go

The full go test ./tikv -count=1 currently hits the same mockstore teardown nil-pointer panic in KVStore.updateSafeTS on the clean #2024 stacked base; the focused changed behavior passes.

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[pingyu]

Superseded by #2040, which implements the CES predicate locally and avoids the API-v3 dependency blast while retaining tikv/pd#11100 as the canonical PD helper.