Implement SubmitError variants

[AbelOsaretin]

Description

Define all SubmitError variants with Display implementations for the transaction submission pipeline. These variants cover every failure mode in the send+poll lifecycle.

Current Behavior

SubmitError in submit.rs has the required variants but needs to be comprehensive and have proper Display impls for logging and error propagation.

Desired Behavior

All variants must be defined with correct Display output:

pub enum SubmitError {
    Rpc(RpcError),           // "RPC error: {inner}"
    JsonError(String),       // "JSON parse error: {msg}"
    Rejected { status },     // "transaction rejected: {status}"
    TransactionFailed { events }, // "transaction failed on-chain; diagnostic events: {events:?}"
    PollTimeout,             // "transaction not confirmed after {MAX_POLL_ATTEMPTS} attempts"
}

Implementation Details

• Each variant wraps the appropriate inner type
• Rpc wraps crate::stellar_rpc::RpcError
• TransactionFailed holds Vec<String> of diagnostic event XDRs
• PollTimeout is a unit variant (no data needed)
• Display must produce human-readable messages for logging
• Add From<RpcError> impl for ergonomic error propagation

Acceptance Criteria

• All 5 variants defined: Rpc, JsonError, Rejected, TransactionFailed, PollTimeout
• Each variant has a Display implementation
• From<RpcError> conversion implemented
• SubmitError derives Debug, PartialEq, Eq, Clone
• Unit test: verify Display output for each variant

[stacyelliott492-star]

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i would love to work on this issues if granted stay safe help the needy

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[Abidoyesimze]

@Abidoyesimze has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi I'd like to work on this issue

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