Enable serde_json float_roundtrip so float payload values survive decode/re-encode #181

Found in the interim review of cli#79, and confirmed on the native Server as server#1010 (fix server#1011, going into Server 3.1.0).

Cargo.toml:138 has serde_json = "1" without the float_roundtrip feature. Without it, serde_json's float parser isn't correctly rounded, so some 16–17-significant-digit doubles decode 1 ULP off. For example, 1.4820142857142857 re-encodes as 1.4820142857142855.

• Measured: about 2.7k of 20k shortest-form doubles mismatched against PHP 8.4 in the CLI fuzz, and 5.8k of 20k changed in a round trip on the Server.
• Impact on the SDK: any float the SDK decodes and re-emits (activity results and arguments, memos, search attributes, heartbeat details, query and update results) can come back with different digits than the client or the PHP/Python SDKs sent. User code that deserializes f64 also sees the off-by-one-ULP value.
• Fix: serde_json = { version = "1", features = ["float_roundtrip"] }. Cargo features are unified across the graph, so this applies to every serde_json user in a worker binary. The only cost is slightly slower float parsing.
• Tests:
  • a round-trip test that decodes and re-encodes [1.4820142857142857, 124435.71428571429, 0.7071067811865476] and gets the digits back unchanged;
  • a payload test through the codec path.
    Each should fail without the feature.

Release: after #178 and 4.11.2, as 4.11.3. If it's ready before 4.11.2's head is frozen, it can go in 4.11.2 instead.