docs(java): add solana x402 payment flow example

[sammccord]

Description

• Adds a new Java example (SignTransactionX402.java) demonstrating the end-to-end x402 payment flow on Solana
• Registers a runSolanaSignTransactionX402 Gradle task to run the example
  What it does
  The example walks through the full x402 Solana payment lifecycle:

1. Query supported x402 payment kinds via the X402FacilitatorApi
2. Create a CDP-managed Solana account as the payer
3. Build an unsigned SOL transfer using the SDK's SolanaTransactionBuilder.buildNativeTransfer()
4. Sign the transaction via CDP (sign-only, no broadcast) — the core x402 pattern where the signed transaction becomes a payment payload rather than being sent directly on-chain
5. Assemble and verify the x402 payment by constructing the v1 payment payload/requirements and calling the facilitator's verify endpoint

Tests

Checklist

A couple of things to include in your PR for completeness:

• Updated the typescript README if relevant
• Updated the python README if relevant
• Added a changelog entry
• Added e2e tests if introducing new functionality

[cb-heimdall]

✅ Heimdall Review Status

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[cb-heimdall]

Review Error for Naughtyknucks @ 2026-03-05 22:51:15 UTC
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[cb-heimdall]

Review Error for Dhfred @ 2026-03-18 17:36:25 UTC
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