test: replace bare asserts with assertion helpers in rpc_blockchain.py #34738

Replace 6 bare assert statements in test/functional/rpc_blockchain.py with test framework assertion helpers (assert_greater_than, assert_greater_than_or_equal, assert_approx).

Bare asserts produce unhelpful AssertionError with no context on failure. The test framework helpers print the actual vs expected values, making debugging much easier.

Changes:

• assert height >= 144 and height <= 287 → two assert_greater_than_or_equal calls
• assert size > 6400 / assert size < 64000 → assert_greater_than calls
• Two assert abs(x - 1) < 0.0001 patterns → assert_approx(x, 1, vspan=0.0001)
• assert hashes_per_second > 0.003 → assert_greater_than
• Added assert_approx to the existing import block

Each replacement preserves the exact same logical condition.

Part of #23119.

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