test: add coverage for feebumper uncomputable cluster error path #35472

Add functional test coverage for the error path in CheckFeeRate() when calculateCombinedBumpFee() returns nullopt because unconfirmed inputs span more than 500 mempool transactions (CTxMemPool::GatherClusters DoS limit).

The crash on this path was fixed in #34870; this PR adds the missing test requested in #34902.

The test builds eight independent 64-transaction clusters (512 total, above the 500 limit), keeps a wallet-owned RBF transaction that spends one output from each cluster out of the mempool, and asserts that bumpfee with an explicit fee_rate returns the expected wallet error instead of proceeding with an empty optional.

Closes #34902.

Testing

• ./build/test/functional/wallet_bumpfee.py
• Temporarily removed the early return in CheckFeeRate() and confirmed the test fails with bad optional_access instead of the expected error message

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35472.

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Reviews

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35442 (test: remove usages of MAX_BIP125_RBF_SEQUENCE constant from functional tests by rkrux)
• #35433 (wallet: deprecate replaceable argument from transaction (and psbt) creation (and modification) RPCs by rkrux)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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@furszy Would you mind taking a look when you have a chance? This adds the functional test coverage for the error path you fixed in #34870.

CheckFeeRate() only runs when the caller passes an explicit fee_rate; default bumpfee uses fee estimation and does not hit this branch, which is why the test passes {"fee_rate": NORMAL}.

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/27053679324/job/79853785327} _{LLM reason (✨ experimental): CI failed because Python linting (py_lint/ruff) reported an error: comment contains an ambiguous × character (RUF003) in test/functional/wallet_bumpfee.py.}

<details><summary>Hints</summary>

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• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

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