feat(email): auto-detect inbox provider for transactional send

[qasim-nylas]

Summary

Automatically detect when a grant uses the Nylas Inbox provider and route email sends through the domain-based transactional endpoint instead of the grant-based endpoint.

User Experience: No changes required - the CLI automatically selects the correct endpoint based on the grant's provider type.

# Works the same for all providers - CLI auto-detects!
nylas email send --to user@example.com --subject "Hello" --body "Hi"

# For inbox provider grants, automatically uses:
# POST /v3/domains/{domain}/messages/send

# For Google/Microsoft/IMAP grants, uses:
# POST /v3/grants/{grant_id}/messages/send

Changes

• New port interface: TransactionalClient for domain-based email sending
• New adapter: SendTransactionalMessage for POST /v3/domains/{domain}/messages/send
• New helper: ExtractDomain() to parse domain from email addresses (e.g., info@qasim.nylas.email → qasim.nylas.email)
• Updated send command: Auto-detects provider type and routes to appropriate endpoint
• Mock/Demo implementations: For testing and demo mode support
• Comprehensive tests: Unit tests for adapter, helper, and mock

Files Changed

File	Description
internal/ports/transactional.go	New port interface
internal/ports/nylas.go	Embed TransactionalClient
internal/adapters/nylas/transactional.go	HTTP client implementation
internal/adapters/nylas/mock_transactional.go	Mock for testing
internal/adapters/nylas/demo_transactional.go	Demo client support
internal/adapters/nylas/transactional_test.go	Unit tests
internal/cli/common/string.go	ExtractDomain helper
internal/cli/common/string_test.go	Helper tests
internal/cli/email/send.go	Auto-detection logic

Test Plan

• Unit tests pass for ExtractDomain helper
• Unit tests pass for SendTransactionalMessage adapter
• Unit tests pass for error handling scenarios
• Mock client implements interface correctly
• Full CI pipeline passes (make ci)
• Manual test with inbox provider grant
• Manual test with Google/Microsoft grant (no regression)