chore: bump version to 3.4.0

[Diego Barajas (barajas-d)]

Summary

• Bump version from 3.3.1 to 3.4.0
• Update pyproject.toml, mercadopago/config/config.py, and CHANGELOG.md
• Note: config.py was on 3.3.0-beta.1 in master — updated directly to 3.4.0

Changelog entry

Added

• SDK ergonomics: typed exceptions, configurable retry, and auto-pagination (#141)
  • MPApiException now has 12 specific subtypes per HTTP status code
  • Request options gain optional max_retries, retry_on, initial_delay_ms, max_delay_ms and on_retry callback
  • New auto-pagination support on search endpoints
• Missing API methods — disbursement_refund.list(), advanced_payment.update(), customer_card.update(), payment.update() (#140)
• CREDENTIAL_ON_FILE messaging fields on Payment types (#139): first_transaction, storage, transaction_initiator, reference

Fixed

• Webhook tolerance_seconds unit mismatch (#142)
• constant_time_equals error on multibyte v1 hash (#142)

Dependencies

• Bump actions/checkout to v7.0.1 (#138)

Test plan

• CI passes
• Version string is correct in all files