fix: remove dead branches in _encode_args and compact length encoder

[aliangm]

Fixes #987

Summary

Removes three dead/unreachable branches in contract_client.py that contained incorrect code and replaced them with assertions that make the invariants explicit.

• Compact encode else branch (_call_contract_method_raw): input_data is always a 4-byte selector so data_len is always 4 — the else (data_len ≥ 64) was never reached. The formula was also wrong: (data_len << 2) | 1 overflows a single byte for data_len ≥ 64, producing a ValueError if ever triggered.
• u128 branch in _encode_args: only register_issue uses u128, but that method is called from the CLI via a different execution path (ContractInstance.exec), never through _exec_contract_raw → _encode_args.
• else: raise ValueError fallback in _encode_args: only str (from register_issue) and unknown (from get_issues_by_status) would reach this branch — neither method is routed through _encode_args.

Changes

• Replace compact encode if/else with assert data_len < 64
• Remove unreachable u128 encoder branch
• Replace raise ValueError fallback with assert False and an explanatory message

Testing

No behavior change for any current caller. The assertions will surface loudly if a future refactor accidentally routes a new method through these paths.

[anderdc]

Branches are unreachable as the issue notes; replacing them with assert is no functional change (asserts strip under -O). Closing.