Encode Mattermost payloads as UTF-8

[DhruvShah-Dev]

What type of PR is this?

Bug fix

Description

Mattermost webhook payloads were passed to requests.post as a Python string, which lets Requests encode the body as latin-1. This matches the Slack destination by sending UTF-8 bytes so non-ASCII alert text is preserved.

Fixes #7775

Validation

• git diff --check
• python -m pytest tests\destinations\test_mattermost.py (blocked locally: missing Redash runtime dependency redis)

[greptile-apps]

Greptile Summary

This PR aligns Mattermost webhook delivery with the Slack destination by explicitly encoding serialized JSON payloads as UTF-8 bytes.

• Encodes Mattermost payloads as UTF-8 before passing them to Requests.
• Adds coverage confirming Unicode alert subjects survive serialization and decoding.

Confidence Score: 5/5

The PR appears safe to merge, with the UTF-8 payload behavior covered by a focused regression test.

The changed request body remains valid serialized JSON while its encoding becomes explicit, matching an established sibling destination pattern without introducing a concrete failure.

Important Files Changed

Filename	Overview
redash/destinations/mattermost.py	Explicitly UTF-8 encodes the serialized Mattermost webhook payload; no actionable regression was identified.
tests/destinations/test_mattermost.py	Adds focused coverage that the submitted body is bytes and preserves a non-ASCII alert subject.

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[cubic-dev-ai]

No issues found across 2 files

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