feat: Return feerate from transaction builder

[gyanranjanpanda]

Implements functionality to return feerate from transaction builder finish() methods.

Fixes #187.

Changes:

• Modified create_tx to return BuilderResult struct containing psbt and fee_rate
• Updated TxBuilder::finish() and TxBuilder::finish_with_aux_rand() to return BuilderResult
• Updated tests and examples to use the new API

This allows callers to know the exact feerate used for the constructed transaction, which is useful for fee estimation and user display.

[notmandatory]

@gyanranjanpanda are you planning to fix the compile errors on this PR? if so please change it to Draft status until you're done. If not we'll close it.

[gyanranjanpanda]

Thanks for the feedback — I’ll fix the compile errors and update the PR shortly.

[gyanranjanpanda]

hi @notmandatory , could u review this wating for your feedback

[ValuedMammal]

Just want to point out that the TxBuilder API is largely stabilized, and that new features should be directed to bdk_tx and the ongoing integration with bdk_wallet.

In terms of this issue, the FeeRate is a trivially copyable type so I don't see what's preventing you from keeping it in a variable alongside your TxBuilder instance. It's not that you lose access to the value after specifying the fee_rate option.

[gyanranjanpanda]

Ah got it thanks for pointing that out. I see what you mean.
My thinking earlier was to return the FeeRate from the builder so it would be easier for callers to access it after setting fee_rate, but you’re right since the API is already stabilized and FeeRate is cheap to copy, changing the builder doesn’t really add much value here.
I didn’t realize that keeping it alongside the TxBuilder instance would already solve the problem cleanly without touching the public API. I’ll rework the PR to follow that approach and keep the changes minimal.
Really appreciate the feedback