Add conviction checks to bridge contract

[silva-fj]

Summary

• Add conviction-aware stake availability checks to the Bittensor bridge contract and Subtensor chain extension binding.
• Enforce available Alpha checks before deposits, withdrawal finalization, and admin manual releases while keeping the existing hotkey stake checks.
• Refresh bridge integration fixtures and coverage, including conviction-locked deposit behavior.
• Update the bridge spec to match the implemented request ID, nonce, and stake-availability behavior.
• Align alpha-bridge pallet tests with existing pallet behavior; no alpha-bridge runtime logic was changed.

Testing

• cargo test -p bridge
• cargo test -p pallet-alpha-bridge
• CONTRACTS_NODE_URL=ws://127.0.0.1:9944 pnpm test:run from contracts/bridge/integration-tests with localnet
• git diff --check

Caution

These changes rely on a new chain extension on subtensor, the contract should not be deployed until those changes land on mainnet. I will open the PR on subtensor once I have everything needed for the Taostats OTC contracts