feat(contact): add validation, tests, types and API docs for contact create api

[ROHITNANERA]

Closes #11

Summary

Adds complete input validation to POST /api/contacts with proper error handling, phone normalization, tags support, and documentation.

Changes

• Required fields: first_name, last_name, email
• Email format + length validation
• Optional phone validated + normalized to E.164 using github.com/nyaruka/phonenumbers
• Optional tags array accepted (matches current API usage)
• All fields trimmed
• Dynamic error messages (max 100 characters)
• Structured 400 JSON error response
• Create types for request and error structure

Tests

• validator_test.go – full unit tests (matches repository test style)
• handler_test.go – integration tests using real in-memory DB + migrations
• All validation cases covered, including error format
• Success cases verify 201 + "id" field (no change to current response design)

Documentation

• Updated apis.rest with:
  • Valid + invalid examples using {{base_url}}
  • Full validation rules
  • Error response format

All tests pass locally:

go test ./internal/contact
# → ok