change(consensus): Optimize checks for coinbase transactions

[upbqdn]

Motivation

The original motivation was to address #9098 (comment) and #9098 (comment). Then I noticed that the coinbase_outputs_are_decryptable fn currently converts each post-Heartwood coinbase tx to check if its shielded outputs are decryptable, even if the tx has no shielded outputs. This conversion involves a serialization and deserialization cycle. We can skip this conversion for txs with no shielded outputs.

Solution

• Return early if the tx has no shielded outputs.
• Return a new Transaction::NotCoinbase error if the tx is not coinbase but should be.
• Update docs.

Tests

• Improve tests so they check more scenarios.

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

[arya2]

Looks good, thank you!