fix(sn-manager): gate unprepared v2.6 upgrades

[mateeullahmalik]

Summary

Build v2.5.3 from the exact v2.5.2 codebase with one sn-manager safety gate:

• block an automatic update from a pre-v2.6 SuperNode to a v2.6+ target when supernode.evm_key_name is missing, blank, or unreadable
• allow the update when evm_key_name is configured
• allow normal forward updates for an already-v2.6 SuperNode when the transitional field is absent

The v2.5.2 base contains no automatic EVM rollback path, so this change does not add or retain one.

Scope

Only sn-manager update gating and focused tests:

• sn-manager/internal/updater/updater.go
• sn-manager/internal/updater/updater_test.go
• sn-manager/internal/utils/chain_id.go
• sn-manager/internal/utils/chain_id_test.go

No release workflow, SuperNode runtime, chain, EVM module, migration, or configuration refactor.

Invariant

Current SuperNode	Target	evm_key_name	Result
<2.6	<2.6	any	unchanged
<2.6	>=2.6	non-empty	allow
<2.6	>=2.6	missing/empty/unreadable	block before download/install
>=2.6	later	missing/empty	allow; never auto-rollback

Prerelease labels do not bypass the boundary: v2.6.0-testnet is treated as a v2.6 binary.

State-machine impact

None.

• State keys: unchanged
• CheckTx / DeliverTx: unchanged
• BeginBlock / EndBlock: unchanged
• Determinism / replay: unchanged
• Migration / upgrade handler: none

Verification

Using Go 1.25.9:

• go test ./... in sn-manager
• go test -race ./internal/updater ./internal/utils
• go vet ./... in sn-manager
• CGO_ENABLED=0 go build ./... in sn-manager
• independent narrow review: approved