feat: add FunctionCallDecoder for Soroban host function invocations (#342)

[JesseJohn7]

Closes #342

Adds a new FunctionCallDecoder module (crates/core/src/decode/function_call_decoder.rs)
that reconstructs the exact function call a Soroban transaction initiated.

• Extracts function_name and raw args (SCVec) from the HostFunction
  invocation
• Resolves the contract's ContractFunction metadata via SpecParser
• Zips the expected parameter list against the raw argument array
• Converts each argument via the existing SCVal-to-JSON converter and
  labels it with its parameter name
• Returns GratError::ArityMismatch (new error variant) with a
  descriptive message when the argument count doesn't match the
  expected parameter count

Also adds GratError::ArityMismatch and GratError::UnsupportedHostFunction
to the error enum, and registers the new module in decode/mod.rs.

[codeZe-us]

@JesseJohn7 please fix workflow issues

[JesseJohn7]

Hi @codeZe-us thanks for the review request. I've resolved the merge conflict in error.rs (the branch now merges cleanly with main).

[codeZe-us]

@JesseJohn7 The workflow is failing, resolve it as well