[demo of #2544] [chore] SDK examples: Java v3.1.0 audit

[kaankacar]

Updating the Java code examples to the most up-to-date version of the SDK available (v3.1.0 at time of writing)

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Demo replay of stellar#2544 (original author: ElliotFriend) — seeded for the triage-agent demo.

[github-actions]

🤖 Claude docs review

READY-FOR-HUMAN-APPROVAL

Mechanics

• ✓ No internal links touched (nothing to check for relative/absolute convention)
• ✓ No headings or image/alt-text changed
• ✓ Diff is Java code-block only; no other languages' samples touched
• ✓ mdx-format check passing; build / review checks were still in progress at review time — worth a glance before merge

Technical accuracy — verified every changed API surface against lightsail-network/java-stellar-sdk tag 3.1.0 (the version this PR claims to target, per the network.lightsail:stellar-sdk:3.1.0 Maven coordinate in docs/build/guides/transactions/create-account.mdx):

• CreateAccountOperation, PaymentOperation, ChangeTrustOperation, SetOptionsOperation, SetTrustlineFlagsOperation, InvokeHostFunctionOperation all live under org.stellar.sdk.operations in 3.1.0, and now use Lombok builder()/build() instead of the old new Foo.Builder(...) constructors — matches the diff.
• AccountNotFoundException, PrepareTransactionException, SorobanRpcException are now under org.stellar.sdk.exception; the old org.stellar.sdk.requests.sorobanrpc.SorobanRpcErrorResponse no longer exists in the 3.1.0 tree — the rename in invoke-contract-tx-sdk.mdx and stellar-transaction.mdx is correct.
• Asset.createNonNativeAsset(...) returns Asset (not AssetTypeCreditAlphaNum12) — the create-account.mdx variable type change is correct.
• ChangeTrustOperation.asset is typed ChangeTrustAsset, and ChangeTrustAsset(Asset) is a real constructor — the new ChangeTrustAsset(astroDollar) wrapping in how-to-issue-an-asset.mdx is correct.
• PaymentOperation.amount, ChangeTrustOperation.limit, CreateAccountOperation.startingBalance are all BigDecimal in 3.1.0 (previously String) — the new BigDecimal("...") changes are correct, and SetTrustlineFlagsOperation.asset remains a plain Asset (not ChangeTrustAsset), which the diff also gets right.
• SetTrustlineFlagsOperation (lowercase "line", renamed from SetTrustLineFlagsOperation) and its trustor/setFlags(EnumSet) fields are accurate.

No inaccuracies found. I did not find a corresponding open issue in this repo tracking a stale Java SDK audit — this PR's own description states it's a demo replay of the upstream stellar#2544 (still open there), seeded for the triage-agent demo, so it isn't closing a tracked item here.

Non-blocking nits

• docs/tokens/how-to-issue-an-asset.mdx's lockAccountTransaction and SetTrustlineFlagsOperation snippets remain illustrative/incomplete (missing outer .build()/imports) — pre-existing in the file, not introduced by this PR, just flagging since it's adjacent to the touched lines.
• Diff is 101 lines (58+/43-) across 6 files, so it's outside the ~10-line trivial-tier threshold regardless of how clean the change is — leaving unlabeled for a human merge decision.