Update upgrade_manager.rs

[LELOUCH-Vii]

Closes #703

Dug into this one and wanted to be upfront about what I found.

The WASM hash chain verifier this issue asks for was already built in #619
and shipped in PR #754 — UpgradeRecord chaining, chain-aware rollback, and
an internal verify_upgrade_chain(env, depth) -> bool in UpgradeManager,
with solid test coverage (valid chains, broken chains, empty history).
#619 is closed and that work is real and correct.

What I found missing: verify_upgrade_chain was never wired into lib.rs
as a public contract entrypoint, unlike every other read-only upgrade query
(get_contract_version, check_upgrade_available, get_upgrade_history,
etc., all of which have a #[contractimpl] wrapper). That means no external
caller — auditor, indexer, frontend — could actually invoke it on a deployed
contract. It was only reachable from tests and from the internal call inside
rollback_upgrade.

That gap is what this PR closes: a public verify_upgrade_chain entrypoint
in lib.rs, plus tests that call it through the actual contract client
(PredictifyHybridClient) instead of the internal module path, to prove it's
reachable end-to-end.

PR: #619

Flagging the overlap with #619/#754 transparently rather than claiming this
as new work from scratch — the verification logic itself was already done;
this closes the "surfaced public" requirement that was specified but not
fully delivered.