refactor!: Simplify request/response system with enum-based approach

[evanlinjin]

Breaking Changes

This PR completely redesigns the request/response API to be simpler and more maintainable.

Summary

Replaces the complex macro-based request/response system with straightforward enums, improving code clarity and reducing complexity.

Changes

Request System

• ✅ Replace macro-generated request types with a single Request enum
• ✅ Inline all request parameters directly in enum variants
• ✅ Remove pending_request.rs and batch_request.rs (no longer needed)

Response System

• ✅ Create unified Response enum for all response types
• ✅ Split ambiguous response types for compile-time clarity:
  • HeaderResp → HeaderResp (simple) and HeaderWithProofResp (with merkle proof)
  • HeadersResp → HeadersResp (simple) and HeadersWithCheckpointResp (with checkpoint)
• ✅ Auto-detect response type in from_json() based on fields present

Error Handling

• ✅ Add clean ClientError enum with just 3 variants: SendError, ServerError, Cancelled
• ✅ Remove complex nested error types from client methods

Client API

• ✅ Direct methods for each request type on both AsyncClient and BlockingClient
• ✅ Type-safe return types (e.g., header() returns HeaderResp, header_with_proof() returns HeaderWithProofResp)
• ✅ Consistent error handling across all methods

Testing

• ✅ Fix blocking client tests to use proper synchronization
• ✅ Remove sleep-based waiting in favor of env.wait_until_electrum_sees_block()
• ✅ Add proper assertions and clean shutdown

Motivation

The previous macro-based system was:

• Hard to understand and debug
• Poor IDE support (no go-to-definition, autocomplete issues)
• Overly complex for the relatively simple Electrum protocol

The new enum-based approach:

• Simple and explicit
• Great IDE support
• Easy exhaustive matching
• Clear request→response relationships

Migration Guide

// Old (macro-generated types)
let req = HeaderRequest { height: 100, cp_height: None };
let resp = client.request(req).await?;

// New (enum-based)
let resp = client.header(100).await?;  // Returns HeaderResp
// or
let resp = client.header_with_proof(100, 50).await?;  // Returns HeaderWithProofResp

Closes #6