test(contract): add tests for token_address validation and get_pool_count sequencing (#1372, #1376)

[Sarietgold]

Summary

This PR implements test suites and validation handling for Soroban smart contract issues #1372 and #1376.

Issue #1372: token_address validation in donate_with_token and refund_donation

• Verified invalid/unregistered token address is rejected before transfer attempt in both donate_with_token and refund_donation.
• Verified non-token contracts fail gracefully without state corruption.
• Enforced token_address validation matching POOL_TOKEN_PREFIX on refund_donation.
• Added tests verifying state integrity against callback/reentrancy.

Issue #1376: get_pool_count sequencing consistency

• Verified initial get_pool_count returns 0.
• Verified count increments by exactly 1 per create_pool and create_pool_for_school call.
• Verified count matches highest valid pool_id usable with get_pool.
• Verified count is unaffected by donation or pool closure.
• Verified consistent count sequence across interleaved admin/school calls.

Closes #1372
Closes #1376
Closes #1374
Closes #1375

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