feat(tracing): instrument Soroban RPC child spans- #1

[lekescrew22]

Closes #857

Summary

Adds explicit child spans for Soroban RPC operations so distributed traces show the RPC hop independently from the surrounding HTTP or GraphQL request.

What changed

• Added reusable async Soroban RPC span helpers to the tracing modules.
• Wrapped every GraphQL Soroban RPC client call:
  • IP lookup
  • Swap lookup
  • Seller/buyer/IP swap listings
  • Dispute evidence lookup
  • Reputation lookup
• Recorded the RPC operation, Soroban system, elapsed duration, and failure status.
• Logged RPC errors while preserving the existing error returned to the caller.
• Added canonical RPC attribute constants in the OpenTelemetry module.

Observability behavior

Each RPC call emits a soroban.rpc span beneath the active request span with:

• rpc.system = stellar_soroban
• rpc.operation = <client operation>
• rpc.duration_ms
• error = true when the RPC returns an error

Compatibility

• No endpoint or response contract changes.
• Existing RPC errors continue to propagate unchanged.
• Cache hits do not create an RPC span because no RPC call occurs.

Validation

• Commit created and branch pushed successfully.
• Rust formatting/tests could not be run in this environment because cargo is not installed.

Review focus

• Confirm the tracing subscriber/OpenTelemetry bridge exports tracing child spans as expected.
• Run cargo fmt --all -- --check and cargo test --manifest-path api-server/Cargo.toml in CI or a Rust-enabled environment.

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