Added collapsed style for issue labeled subscriptions mattermost/matt…

[JOAO-Ethan]

Summary

Added a collapsed style for labeled issue notifications.

Ticket Link

Fixes #740

[mattermost-build]

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Pull request overview

This PR adds a collapsed notification style for GitHub issue labeled events. When the collapsed style is configured, issue labeled notifications display in a more compact format instead of the default expanded format with title and multiple lines.

Key Changes:

• Modified the issueLabelled template to support both collapsed and expanded notification styles

Comments suppressed due to low confidence (2)

server/plugin/template.go:1

• There are two spaces between the closing backtick and 'by'. This should be reduced to one space for consistent spacing.

// Copyright (c) 2018-present Mattermost, Inc. All Rights Reserved.

server/plugin/template.go:1

• The word 'issue-labeled' uses a hyphen while line 311 uses 'labeled' without 'issue-' prefix. Consider using consistent terminology across both style variants (either 'labeled' or 'issue-labeled' in both places) for better maintainability.

// Copyright (c) 2018-present Mattermost, Inc. All Rights Reserved.

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[nevyangelova]

Thanks @JOAO-Ethan looks good to me. Note for QA:

1. Create a subscription with --render-style collapsed /github subscribe owner/repo issues --render-style collapsed
2. Label an issue in GitHub
3. Verify the notification in Mattermost is a single line like:

owner/repo issue #123 labeled bug by @username.

4. Then repeat with expanded style /github subscribe owner/repo issues --render-style expanded
5. Verify the notification shows heading format:

Issue Title

owner/repo#123

#issue-labeled enhancement by @username.

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