feat: add transfer_fungible_token tool to core_token_plugin

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

JS reference: [PR #1024](https://github.com/hashgraph/hedera-agent-kit-js/pull/1024)

## Scope

- Add `TransferFungibleTokenTool` to `core_token_plugin`
- Mirror JS parameter schema: token id, sender account id, receiver account id, amount
- Enforce HTS int64 limit on amount (see T2 — add shared `validate_hts_amount` guard)

Start in the core_token_plugin and compare the requested parameters with JS reference PR #1024. Confirm that predicates epic #353 is complete, then trace T2's shared validate_hts_amount guard. Done means the tool accepts token, sender, receiver, and amount parameters and enforces the HTS int64 amount limit.

Written by the indexing model from the issue text.