Fix #25980: Validate transactions in combinerawtransaction #33361

Summary

This PR fixes issue #25980 where combinerawtransaction would silently accept unrelated transactions and only use the first one, ignoring the rest.

Problem

When passed two or more unrelated transactions (transactions with different inputs/outputs), combinerawtransaction would:

• Take the first transaction as the base
• Silently ignore the unrelated parts of subsequent transactions
• Only merge signatures for matching inputs

This behavior was confusing and could lead to unexpected results where users think they're combining transactions but actually only getting the first one back.

Solution

Added validation to ensure all transactions being combined are actually the same transaction (with potentially different signatures). The validation checks that all transactions have:

• The same number of inputs and outputs
• Identical input outpoints and sequences
• Identical output values and scripts
• The same version and locktime

If any transaction differs from the first, the RPC now throws a descriptive error indicating which transaction is unrelated.

Testing

Added test coverage in test/functional/rpc_rawtransaction.py that verifies:

1. Unrelated transactions are properly rejected with an appropriate error message
2. Related transactions (same transaction structure) can still be combined successfully

Fixes #25980

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33361.

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Reviews

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#31298 also fixes this issue.

CI is failing because you've added a test that uses wallet behavior to a non-wallet test.

I'm currently favoring the fix in #31298, as it avoids comparing all the involved transaction fields (and input/output counts) by using the txid instead and is thus much simpler. One potential advantage of the approach of this PR though is that the error messages are more detailed, pointing out where exactly the txs differ -- personally I think just pointing out that the txs are not compatible is sufficient.

Closing for now. This is LLM generated and obviously wrong (the tests fail), and the author does not seem to be working on it (no activity since this was opened 2 weeks ago)

In the future, instead of creating competing pull requests, it would be better to just review the existing pull request with any feedback you may have.