[jdjioe5-cpu]

Summary

Adds the stellar tx send-via-core subcommand to submit base-64 encoded XDR transaction envelopes directly to a stellar-core HTTP endpoint (/tx?blob=...), bypassing Soroban RPC.

Closes #2391

Changes

• cmd/soroban-cli/src/commands/tx/send_via_core.rs:
  • Implements stellar tx send-via-core [TX_XDR] --core-url <CORE_URL> (also configurable via STELLAR_CORE_URL).
  • Accepts transaction envelope XDR from a positional argument, a file path, or stdin (reusing super::xdr::tx_envelope_from_input).
  • Encodes the transaction envelope as canonical base-64 XDR and constructs the GET <core-url>/tx?blob=<url-encoded-xdr> request URL.
  • Parses and pretty-prints the JSON response from stellar-core (status, optional error, and any additional diagnostic fields).
• cmd/soroban-cli/src/commands/tx/mod.rs:
  • Registers the send-via-core subcommand under stellar tx.

Testing

• Added unit and end-to-end mock server tests in commands::tx::send_via_core::tests covering URL construction, CLI argument parsing, response deserialization, and HTTP submission to /tx?blob=....