Python-v2: async lifecycle hooks

[phdargen]

Description

PR #900 introduced a dual class design for x402HTTPResourceServer to support async dynamic route hooks.

This PR follows the same strategy to also support async server/client/facilitator lifecycle hooks.
Async classes should be considered default offering the best features (i.e. async callbacks), with the Sync classes as fallback for codebases or frameworks that can't use async.

Requests client and flask middleware use the Sync classes
Httpx client and fastapi use the Async classes

Shared base logic of sync/async classes is refactored into X_base.py with sync/async specifics in X.py using generator based approach to reduce excessive code duplication.

Async (default)	Sync	Location
x402Client	x402ClientSync	client.py
x402ResourceServer	x402ResourceServerSync	server.py
x402Facilitator	x402FacilitatorSync	facilitator.py
x402HTTPResourceServer	x402HTTPResourceServerSync	http/x402_http_server.py
HTTPFacilitatorClient	HTTPFacilitatorClientSync	http/facilitator_client.py
x402HTTPClient	x402HTTPClientSync	http/x402_http_client.py

Other updates:

• adds missing unit tests for core, http/clients and http/middleware
• adds new integration test to verify HTTP client/server integration (async & sync)
• adds new custom server example
• adds new advanced server example showing how to use the paywall builder
• updates and cleans-up all examples
• adds utility function that takes care of html-safe escaping in paywall
• added sdk READMEs and removed implementation plans

Tests

• Manually tested all examples
• e2e tests:

╔════════════════════════════════════════════════════════╗
║         Bazaar Discovery Extension Validation          ║
╚════════════════════════════════════════════════════════╝

📋 Expected Discoverable Endpoints: 5

═══════════════════════════════════════════════════════
                 Discovery Summary
═══════════════════════════════════════════════════════
Total Facilitators:          1
Facilitators with Bazaar:    1
Expected Endpoints:          5
Total Discovered Resources:  5

📍 python:
   Discovered: 5/5
   ✅ All expected endpoints discovered

═══════════════════════════════════════════════════════
✅ Discovery Validation: PASSED
═══════════════════════════════════════════════════════


🧹 Cleaning up...
  🛑 Stopping facilitator: python

📊 Test Summary
==============
✅ Passed: 7
❌ Failed: 0
📈 Total: 7

📋 Detailed Test Results
========================

✅ PASSED TESTS:

  # 1: httpx → fastapi → /protected
      Facilitator: python | Network: eip155:84532 | Tx: 2176556d56...
  # 2: httpx → fastapi → /protected-2
      Facilitator: python | Network: eip155:84532 | Tx: 56e8156df1...
  # 3: httpx → fastapi → /protected-svm
      Facilitator: python | Network: solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1 | Tx: UATvrQnUas...
  # 4: requests → fastapi → /protected
      Facilitator: python | Network: eip155:84532 | Tx: 04a97d14e7...
  # 5: requests → fastapi → /protected-svm
      Facilitator: python | Network: solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1 | Tx: 4PGH4WmVt1...
  # 6: httpx → flask → /protected
      Facilitator: python | Network: eip155:84532 | Tx: 953fae4950...
  # 7: httpx → flask → /protected-svm
      Facilitator: python | Network: solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1 | Tx: 4w9N8Keonh...

📊 Breakdown by Facilitator:
   python          ✅ 7 / ❌ 0 (100%)

📊 Breakdown by Server:
   fastapi              ✅ 5 / ❌ 0 (100%)
   flask                ✅ 2 / ❌ 0 (100%)

📊 Breakdown by Client:
   httpx                ✅ 5 / ❌ 0 (100%)
   requests             ✅ 2 / ❌ 0 (100%)

📊 Protocol Family Breakdown:
   EVM: ✅ 4 / ❌ 0 / 📈 4 total
   SVM: ✅ 3 / ❌ 0 / 📈 3 total

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

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