test: allow 3 bytes of fee overestimate in assert_fee_amount #36408

The wallet sizes a transaction assuming a 71-byte low-R ECDSA signature. A signature whose r or s has leading zero bytes is shorter, so the fee can cover a few bytes more than the signed transaction needs. In #36394 the wallet estimated 219 bytes and paid 438 sat at 2 sat/vB, but the signed transaction was 216 bytes: the signature was 3 bytes short. That happens about once in 4 million signatures. assert_fee_amount allows only 2 bytes of overestimate, so the test failed.

I raise the allowance to 3 bytes, which makes the failure about 200 times rarer. The check that the fee is not too low is unchanged. @maflcko suggested 3 bytes in #24151 and #25164.

Fixes #36394.

Made with my usual tools: a computer, the Internet and an LLM. The mistakes, as usual, are all mine.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36408
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36408.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36408#author-ralph

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tACK 050409b2c5522eff0dc37c00a6d1225a8a437e17

Tested on macOS with Python 3.14. wallet_send.py passes on this commit.

I read through #36394 and the CI log there. Since running the test won't reproduce it (a 3-byte short signature is roughly 1 in 4 million), so I forced it instead by bumping the signature size in PKHDescriptor::MaxSatSize() from 71 to 74. That makes the wallet overestimate by 3 bytes on every run.

On master with that change, wallet_send.py fails at the fee_rate=7 check:

AssertionError: Fee of 0.00001554 BTC too high! (Should be 0.00001533 BTC)

With 73 (+2) instead, the old check passes, as expected.

With this PR and the same +3 change, all the assert_fee_amount checks pass. One more thing I noticed, with the +3 change, the run gets past all the fee checks but then fails in test_maxfeerate() with this error:

 test_framework.util.JSONRPCException: Fee rate exceeds maximum configured by user (maxfeerate) (-6)  [http_status=200]

This is most likely from my +3 change but I thought it's worth noting.