feat: add CUSTOM_EXECUTE_TX / CUSTOM_RETURN_BYTES modes and pluggable TransactionStrategy

#353 (predicates epic) must be fully complete before starting this task.

JS reference: [PR #986](https://github.com/hashgraph/hedera-agent-kit-js/pull/986)

## Scope

- Add `CUSTOM_EXECUTE_TX` and `CUSTOM_RETURN_BYTES` to `AgentMode` enum
- Define `TransactionStrategy` abstract protocol as public API
- Allow `Context` to accept a user-supplied `strategy: TransactionStrategy`
- Export `TransactionStrategy`, `ReturnBytesStrategy`, `handle_transaction` from the core package
- Add `docs/TRANSACTION_MODES.md` documenting all modes and the custom-strategy pattern

Start by reading the existing AgentMode enum, Context, and transaction-handling entry points, then compare the JS reference in PR #986. Define the public strategy API and custom modes, wire user-supplied strategies through Context, and update the core exports. Done includes docs/TRANSACTION_MODES.md covering every mode and the custom-strategy pattern, with predicates epic #353 complete first.

Written by the indexing model from the issue text.