fix(antd-kotlin): make put-response cost optional; ship missing gradle wrapper

[Nic-dorman]

Fixes #69.

Two issues blocked antd-kotlin example 02 from running. Both fixed here because either one alone leaves the binding non-functional out of the box.

1. DataPutPublicDto deserialization failure

The example fataled on:

Caused by: kotlinx.serialization.MissingFieldException:
  Field 'cost' is required for type with serial name
  'com.autonomi.sdk.DataPutPublicDto', but it was missing
        at com.autonomi.sdk.AntdRestClient$dataPutPublic$$inlined$postJson$1.invokeSuspend(AntdRestClient.kt:284)

The daemon currently returns {address, chunks_stored, payment_mode_used} for PUT /v1/data/public — no cost field:

$ curl -s -X POST http://127.0.0.1:8082/v1/data/public \
       -H 'Content-Type: application/json' \
       -d '{"data":"SGVsbG8gd29ybGQ="}'
{"address":"5c0…666","chunks_stored":3,"payment_mode_used":"single"}

Make cost default to empty in DataPutPublicDto (and DataPutPrivateDto), mirroring how the PHP REST client falls back to '' and how the Python REST client uses .get(...) with implicit None. Non-breaking — PutResult.cost still ends up the same String type downstream.

2. Missing gradle wrapper files

antd-kotlin/ shipped gradle/wrapper/gradle-wrapper.properties but not gradlew or gradle/wrapper/gradle-wrapper.jar. The README tells users to run ./gradlew :examples:run --args=2, which gives:

bash: ./gradlew: No such file or directory

Add the wrapper script (executable) and the wrapper jar, copied from antd-java/. gradle-wrapper.jar is a checked-in binary in both antd-java/ and other JVM projects in this repo, so adding it here is consistent.

Test plan

• bash ./gradlew :examples:build --no-daemon -q — succeeds, downloads gradle 8.8 per wrapper config
• bash ./gradlew :examples:run --args=2 --no-daemon -q against a local devnet — exits 0, prints address, retrieved data, "Public data round-trip OK!"
• Verified via the cross-SDK e2e harness (the very thing that surfaced antd-kotlin example 02 fails: DataPutPublicDto requires field 'cost' but daemon response omits it #69)

Sample output

=== Example 02: Public Data ===
Estimate: 16 bytes in 3 chunks, storage 0 atto, gas 0 wei, mode single
Stored at address: 01b06330e4f4a8c565087e4087caabf860bdadbe96b7b8f0447493b0081c2335
Actual cost:  atto tokens
Retrieved: Hello, Autonomi network!
Public data round-trip OK!

(Actual cost blank for the same reason — daemon doesn't return it on this path. That's worth its own ticket if cost is supposed to be populated on PUT responses; the SDK-side change in this PR is correct either way since "optional with default" matches reality.)