Add migration guide for seam v2 to v3 upgrade

[razor-x]

Summary

Add comprehensive migration documentation for upgrading from seam v2 to v3 of the Python SDK. This guide covers all breaking changes, new features, and provides a step-by-step migration checklist.

Changes

• New MIGRATION.md file with detailed upgrade instructions covering:
  • Installation instructions for v3 prerelease versions
  • Summary table of breaking changes with impact assessment
  • Python 3.11+ requirement explanation
  • HTTP library migration from niquests to httpx with code examples
  • Retry configuration changes using httpx-retries
  • Client-side parameter validation behavior
  • Removal of lts_version attribute
  • HTTP method changes (GET for reads, PATCH/PUT for updates, DELETE for deletes)
  • New features including explicit null support with NULL sentinel
  • Complete migration checklist with 9 actionable steps
  • Separate section for v1 to v2 migration for users still on older versions

Notable Details

• Includes side-by-side code examples showing v2 vs v3 patterns for key changes
• Documents the new default retry policy (3 attempts with exponential backoff)
• Explains the distinction between None (omit parameter) and NULL (explicit null)
• Provides guidance on updating proxies, firewalls, and test mocks for new HTTP methods
• Includes helper function exports like serialize_url_search_params for custom HTTP clients

https://claude.ai/code/session_01GK2tFMid8Mus3fsRRwgKBY