feat(calldata)!: migrate method-call key "method" -> "" (v0.6 genvm ABI)

[MuncleUscles]

Migrates genlayer-py to the GenVM v0.6 calldata ABI, which renames the
method-call key from "method" to "" (empty string). Because the calldata
encoder sorts map keys ascending, "" sorts first — the method name becomes
the binary prefix of the encoded calldata. genvm-manager auto-remaps
"method" -> "" but logs an error on every call, so the SDK must migrate.

SDK-side counterpart of the wire-format change already merged in genlayer-node
(#1502) and genlayer-studio (#1697).

Wire-format change

Before: calldata.encode({"args": [...], "method": "foo"})
After: calldata.encode({"args": [...], "": "foo"})

The empty key sorts first in canonical calldata encoding, so for a call to
ask_for_coin(...) the raw calldata now begins with the method-name bytes:

old: 16 04 61 72 67 73 ... 06 6d 65 74 68 6f 64 64 61 73 6b 5f 66 6f 72 5f 63 6f 69 6e   (…"method"·ask_for_coin)
new: 16 00 64 61 73 6b 5f 66 6f 72 5f 63 6f 69 6e 04 61 72 67 73 ...                      (·""·ask_for_coin·"args"…)

What changed

• genlayer_py/contracts/utils.py — make_calldata_object, the sole
  producer of the method key, now emits ret[""] = method. This is the only
  source change; it flows through every method-call path (write / read / sim /
  estimate + encode_tx_data_call).
• Encoder/decoder unchanged — both are key-agnostic. The decoder
  round-trips whatever key is present, so decoded calldata now exposes the
  method name under "" with no decoder change.
• Tests — consensus + smoke round-trip assertions read call_data[""];
  new test_method_key_is_empty_string_binary_prefix asserts the "" key is
  present, "method" is absent, and the method-name bytes precede the "args"
  key bytes in the raw calldata (proving "" sorts first). This test fails
  under the old key.
• Fixtures — regenerated the raw / base64 / readable calldata sample data
  ({"": "ask_for_coin", ...}) in the write sample-data files and conftest.
  Deploy (constructor) fixtures are unaffected — they carry no method key.

Compatibility

No back-compat toggle: genlayer-py has no calldata versioning mechanism, and
none is introduced (matching node/studio). Calldata built by this SDK is
incompatible with pre-v0.6 GenVM.

BREAKING CHANGE: method-call calldata now keys the method name under ""
instead of "method".

Testing

uv run --python 3.13 pytest tests/unit -m "not testnet" -> 91 passed, 17
deselected (testnet-marked tests need live RPC). Verified the new property
test fails when reverted to the old "method" key.

Depends-On: genlayerlabs/genvm-manager#1
Depends-On: https://github.com/genlayerlabs/genlayer-node/pull/1502
Depends-On: https://github.com/genlayerlabs/genlayer-dev-env/pull/95
Depends-On: https://github.com/genlayerlabs/genlayer-e2e/pull/619
Depends-On: genlayerlabs/genlayer-studio#1697
Depends-On: #96
Depends-On: genlayerlabs/genlayer-js#192
Depends-On: genlayerlabs/genlayer-testing-suite#98

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