test: Replace stale TODO with mempoolminfee test in wallet_send.py #36705

wallet_send.py had a commented-out TODO from the original send PR (#16378) about returning the hex when the fee rate is below the mempool minimum. The commented code was stale; estimate_mode="sat/b" no longer exists.

Also, an explicit fee_rate is only checked against max(-mintxfee, -minrelaytxfee), not against mempoolminfee (see src/wallet/fees.cpp), so when the mempool is full, send can build a tx the node won't accept.

This PR adds a subtest covering this cases and removes the stale TODO.

With add_to_wallet=false, the returned hex is rejected by testmempoolaccept, and with add_to_wallet=true, send reports success, but the tx stays only in the wallet, not the mempool.

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36705
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36705.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36705#author-ralph

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline and AI policy for information on the review process. A summary of reviews will appear here.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

LLM Linter (✨ experimental)

Possible typos and grammar issues:

• test/functional/wallet_send.py: Cwallet::CommitTransaction → CWallet::CommitTransaction [class name capitalization typo]

^{2026-10-30 13:04:50}