feat(python-sdk): add webhook verification and event handling (usesen…

…d#344)

* feat(python-sdk): add webhook verification and event handling

Add webhook support to the Python SDK matching the JS SDK implementation:
- Add Webhooks class with verify() and construct_event() methods
- Implement HMAC-SHA256 signature verification with timing-safe comparison
- Add timestamp validation with configurable tolerance (default 5 minutes)
- Add comprehensive webhook event types (18 events: email, contact, domain, test)
- Add WebhookVerificationError with typed error codes
- Export webhook constants (headers) and types

* fix(python-sdk): harden webhook parsing and typing

Normalize invalid UTF-8 webhook payloads to INVALID_BODY errors so verify() safely returns false, and narrow base email webhook event types to avoid discriminated-union overlap. Add regression tests for both paths.

* chore(python-sdk): bump package version to 0.2.9

* feat(python-sdk): add local webhook test example project

Add a runnable Flask receiver and signed webhook sender under packages/python-sdk/example, and link it from the Python SDK README for local verification.

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Co-authored-by: Claude <noreply@anthropic.com>

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