feat: context propagation and dynamic type analysis

[subomi]

Summary

This PR introduces two major enhancements to the requestmigrations library:

1. Context Propagation - New rm.For(r) pattern that makes request context required by design
2. Runtime Type Detection - Support for migrations on interface{} fields where concrete types are determined at runtime

Changes

Context Propagation (rm.For(r) Pattern)

• New Migrator type: A request-scoped handle created via rm.For(r) that holds the HTTP request context and user version
• Updated API: Marshal/Unmarshal now live on Migrator instead of RequestMigration
• Updated TypeMigration interface: Now accepts context.Context parameter for cancellation, logging, and tenant-specific logic
• New helper function: UserVersionFromContext(ctx) to retrieve user version inside migrations

Before:

rm.WithUserVersion(r).Unmarshal(data, &v)  // Request was optional
rm.Marshal(v)  // Could skip migrations silently

After:

migrator, err := rm.For(r)  // Request is required
migrator.Unmarshal(data, &v)
migrator.Marshal(v)

Runtime Type Detection for Dynamic Fields

• New BuildFromValue method: Inspects runtime types before JSON serialization to detect concrete types in interface{} fields
• Enables migrations on wrapper types: Works with patterns like PagedResponse{Content: []EndpointResponse{}}
• Cycle detection: Uses pointer-address tracking to handle recursive structures

Documentation

• Added docs/context_propagation.md - Design doc for the rm.For(r) pattern
• Added docs/runtime_type_detection.md - Design doc for dynamic field support
• Added docs/bulk_registration.md - Design doc for bulk migration registration

Tests

• Updated all existing tests to use new rm.For(r).Marshal/Unmarshal pattern
• Added tests for interface{} field migrations
• Added tests for nested interface slices
• Added tests for context propagation and UserVersionFromContext()

Breaking Changes

• TypeMigration interface signature changed: methods now require context.Context as first parameter
• Marshal/Unmarshal moved from RequestMigration to Migrator
• WithUserVersion() replaced by For()

Test Plan

• All existing tests updated and passing
• New tests for interface field migrations
• New tests for context propagation
• Examples updated to demonstrate new API