Normalize stacktraces and sanitize data for safer storage

[pavelzotikov]

Resolves #39.

This PR adds stacktrace normalization and data sanitization utilities to ensure safer and more consistent error payloads before storing them on the server.

[Copilot]

Pull Request Overview

This PR introduces stacktrace normalization and data sanitization utilities to ensure safer and more consistent error payloads before server storage, addressing issue #39.

• Adds comprehensive stacktrace normalization to standardize frame formats and handle various input types
• Implements data sanitization for MongoDB compatibility by transforming problematic keys and values
• Updates GitHub Actions cache version for improved CI reliability

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/EventPayloadBuilder.php	Adds three new methods for stacktrace normalization, key sanitization, and JSON transformation
.github/workflows/ci.yaml	Updates cache action from v2 to v4

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