Fix JSON escaping in Slack notifications

[gcasilva]

Description of the issue

Slack notification payloads interpolate issue and pull request metadata directly inside JSON string literals. Titles containing double quotes, backslashes, or newlines can therefore produce invalid JSON and cause the notification action to fail.

Fixes #2147.

Description of changes

Use GitHub Actions' toJSON function for every dynamic payload string in both the issue and pull request notifications. The expression engine now emits correctly escaped JSON values before the Slack action parses the payload.

License

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Tests

• actionlint v1.7.12 on .github/workflows/slack-notification.yml
• YAML parse and payload replay using the exact quoted title from issue nvidia_smi input reports 'Unknown schema version "v13"' #2146
• Payload replay with double quotes, backslashes, and newlines for both event types
• git diff --check

Requirements

No Go or shell source files are changed; workflow-specific validation is listed above.

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Integration Tests

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