fix(test): wait for block template reset in RpcTruncate

[eval-exec]

What problem does this PR solve?

Problem Summary:
https://github.com/nervosnetwork/ckb/actions/runs/21896507826/job/63217348878?pr=5107
RpcTruncate integration test can fail in CI after truncate when subsequent mining still follows a stale block-template parent from the pre-truncate tip, leading to cell1 staying unknown instead of live.

What is changed and how it works?

What's Changed:

• Add a guard in test/src/specs/rpc/truncate.rs to wait until block-template parent_hash switches to the truncated tip.
• Keep the existing tx-pool clear assertions and flow unchanged.

Related changes

• PR to update owner/repo: N/A
• Need to cherry-pick to the release branch: No

Check List

Tests

• Integration test (CI)

Side effects

• None expected

[Copilot]

Pull request overview

This PR stabilizes the RpcTruncate integration test by ensuring the node’s block-template parent is refreshed to the post-truncate tip before proceeding, preventing mining from using a stale pre-truncate parent and avoiding incorrect cell state assertions.

Changes:

• Add a blocking wait after truncate until get_block_template reports parent_hash == to_truncate.
• Keep the existing tx-pool synchronization and assertions unchanged.

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