Fix Mattermost notification attachment colors

What does this merge request do and why?

Expands the three-digit Mattermost attachment color #345 to its equivalent six-digit value, #334455.

Mattermost only accepts named styles or six-digit hexadecimal colors, so the existing value produces validation warnings.

The affected chat-message specifications have been updated, with a focused regression assertion for the generated attachment color.

Closes #590632

Testing

• Reproduced the Mattermost color validation behavior.
• git diff --check passes.
• Ruby syntax checks pass with Ruby 3.3.11.
• Focused RSpec tests were not run locally because the project dependencies are unavailable in this sparse checkout; CI should run them.

MR acceptance checklist

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