Feedback: x402 support and hiero-sdk-python 0.2.6 compatibility

## Feedback on Hedera Agent Kit (Python)

### What I built
HashA2A — an Agent-to-Agent Intelligence Layer on Hedera with:
- MCP Server (10 tools) for AI agents
- x402 dual rail payments (Base USDC + Hedera HBAR/HTS)
- Multi-oracle aggregation (Pyth, CoinGecko, DeFiLlama)
- A2A Protocol (Google A2A compliant)

### What worked well
- The SDK docs for topic creation and HCS messaging were clear
- HIP-991 custom fee setup was straightforward
- Mirror node API queries worked reliably

### Suggestions

1. **x402 support**: It would be great to have native x402 payment flow helpers in the agent kit — currently we built X402HederaHandler from scratch with signature verification, nonce checking, and HTS token validation. A built-in x402 facilitator would lower the barrier for agent commerce.

2. **hiero-sdk-python 0.2.6 compatibility**: The agent kit currently pins `hiero-sdk-python==0.2.0` which conflicts with the latest SDK (0.2.6). Updating the dependency range to `>=0.2.0,<0.3.0` would allow projects to use the latest SDK features (like the improved `TransferTransaction` and `TopicMessageQuery`).

3. **Simple Fees (HIP-1261)**: Documenting that `transaction_fee` is a MAX cap (not actual charge) and recommending 0.05-0.1 HBAR for HCS operations would save developers from overpaying.

### Repo
https://github.com/ymiydev-prog/HashA2A

### Bounty submission
Hedera AI Agent Bounty — Week 3 (MCP or x402 Agent)

No source files, tests, or entry points are named. First confirm with maintainers which of the three suggestions is in scope, then locate the dependency declaration or relevant payment and documentation areas. Done should be defined as one selected change being implemented and validated without expanding into the other proposals.

Written by the indexing model from the issue text.