test(python/tvm): add unit tests for codecs/common.py

[0xAxiom]

Add 41 unit tests for python/x402/x402/mechanisms/tvm/codecs/common.py — the only file in mechanisms/tvm/codecs/ without a dedicated unit-test module after the TVM SDK landed in #1944.

Coverage

All 8 helpers in codecs/common.py:

• normalize_address — raw wc:hex passthrough, Address object input, user-friendly to raw conversion, parity between string and Address inputs, invalid-address rejection (AddressError).
• address_to_stack_item — slice typing ({"type": "slice", "value": <base64>}), base64 BoC round-trip via Cell.one_from_boc, user-friendly input acceptance.
• decode_base64_boc — round-trip with encode_base64_boc, non-string rejection covering bytes/None/int with ValueError("base64-encoded BoC string"), invalid-base64 rejection.
• make_zero_bit_cell — Cell instance type, single zero-bit payload (remaining_bits == 1, load_bit() == 0), determinism across calls, exact byte parity with manual Builder().store_bit(0).end_cell().
• encode_base64_boc — string return type, exact byte parity with cell.to_boc(), round-trip with richer cells (uint + address).
• get_network_global_id — mainnet tvm:-239 → -239, testnet tvm:-3 → -3, positive ID tvm:42 → 42, missing/uppercase prefix rejection (ValueError("Unsupported TVM network")), non-numeric payload rejection.
• parse_amount — decimal-to-smallest-unit, zero, zero-decimals, sub-unit truncation via int(), nine-decimal native TON amounts (1 → 1_000_000_000, 0.000000001 → 1), InvalidOperation on non-numeric.
• parse_money_to_decimal — int/float passthrough, $ prefix, USD/USDT and lowercase suffixes with and without separating whitespace, combined $ ... USD prefix+suffix, surrounding whitespace, float type guarantee on string inputs, ValueError on unparseable strings.

Verification

• uv run pytest tests/unit/mechanisms/tvm/test_common.py -q → 41 passed in 0.11s
• uv run pytest tests/unit/mechanisms/tvm/ -q → 229 passed in 0.69s (no regressions in existing TVM suite)
• uv run ruff format clean, uv run ruff check clean
• Commit GPG-signed (RSA 7C3D4BFE44B598C90CE31DCCA190D7A3B1700D63)

Test file follows the established TVM convention: pytest.importorskip("pytoniq_core") so the suite skips cleanly when the optional [tvm] extras are not installed.

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