test: replace bare asserts with assertion helpers in wallet_bumpfee.py #34739

Replace 6 bare assert statements in test/functional/wallet_bumpfee.py with test framework helpers (assert_greater_than, assert_equal).

Bare asserts produce unhelpful AssertionError with no context on failure. The helpers print actual vs expected values.

Changes:

• assert bumped_tx["fee"] > -rbftx["fee"] → assert_greater_than
• assert bumped_psbt["fee"] > -rbftx["fee"] → assert_greater_than
• assert len(oldwtx["walletconflicts"]) > 0 → assert_greater_than
• assert len(bumpedwtx["details"]) == 1 → assert_equal
• assert bumpedwtx["details"][0]["address"] == new_address → assert_equal
• assert rbf_node.getbalance() < 49 → assert_greater_than(49, ...)

No new imports needed — all helpers already imported. Each replacement preserves the exact same logical condition.

Part of #23119.

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lgtm ACK 73bd855c1b430d3630ca7b2de283a9a8a35f44d9