wallet: Report importdescriptors timestamp errors per item #35253

Fixes #35181

This updates importdescriptors to return per-request errors for missing or invalid timestamp fields instead of aborting the whole batch. Requests that fail timestamp validation are skipped, while later valid requests can still be processed.

The rescan result handling now preserves existing per-item errors before checking request timestamps.

Testing:

• cmake --build build --target bitcoind bitcoin-cli -j 8
• test/functional/wallet_importdescriptors.py --configfile=/home/carl/bitcoin/build/test/config.ini --portseed=35181
• git diff --check

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35253.

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Reviews

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The testing instructions are wrong in a flavor that is typical for LLM PR submissions, which makes me think this PR and its description were not understood by the developer writing it. We require developers to understand the changes they are submitting, since review otherwise turns into reviewers prompting the developers LLM. Closing.