Show diagnostic events when a transaction fails on-chain

[fnando]

What

When a transaction passes simulation but fails on-chain, the CLI now prints the transaction's diagnostic events (host/contract error, message, and any logs) on stderr, across contract invoke, tx send, and the tx op commands. Previously only the TransactionResult result codes were shown. Closes #495.

Why

The reason for a failure was never displayed — the diagnostic events were decoded and available on the RPC response but discarded before reaching the user.

Before (main):

🌎 Sending transaction…
❌ error: transaction submission failed: Some(
    TransactionResult {
        fee_charged: 10858,
        result: TxFailed(
            VecM([OpInner(InvokeHostFunction(ResourceLimitExceeded))]),
        ),
        ext: V0,
    },
)

After (this branch):

🌎 Sending transaction…
❌ Error event: [{"symbol":"error"},{"error":{"budget":"exceeded_limit"}}] = {"vec":[{"string":"operation instructions exceeds amount specified"},{"u64":"3880"},{"u64":"1"}]}
❌ error: transaction submission failed: Some(
    TransactionResult { ... InvokeHostFunction(ResourceLimitExceeded) ... },
)

The new Error event line surfaces the host's message (operation instructions exceeds amount specified, needs 3880 vs 1 allowed) — absent before.

Known limitations

Only Soroban transactions have diagnostic events; classic ops print nothing extra. The event line uses raw ScVal JSON rather than fully decoded text.

[Copilot]

Pull request overview

Surfaces Soroban diagnostic events when on-chain transaction submission fails.

Changes:

• Adds shared failure-event recovery after polling.
• Formats error, contract, and log diagnostics.
• Applies behavior to invoke, send, and transaction operation commands.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
cmd/soroban-cli/src/tx.rs	Adds shared submission and event recovery.
cmd/soroban-cli/src/log/event.rs	Formats failed transaction diagnostics.
cmd/soroban-cli/src/commands/tx/send.rs	Uses enhanced submission for tx send.
cmd/soroban-cli/src/commands/tx/args.rs	Uses enhanced submission for transaction operations.

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[leighmcculloch]

Couple questions otherwise looks good to me.