Display summary provided by DCR when it has nothing specific to comment on

Problem

Duo currently say I finished my review and found nothing to comment on. Nice work! :tada: when it had nothing specific to comment on. However, Duo sometimes provides a general summary of what it reviewed. It makes the user experience more positive as it provides more context and give user an assurance that Duo actually reviewed the MR so we should display that.

Example:

<review> The change looks good. The MR adds the environment variable `export GITLAB_SIMULATE_SAAS=0` to the documentation for setting up a cloud license via CustomersDot. This aligns with the MR description which mentions that this variable is needed when running a self-managed instance to add subscriptions. The change is straightforward and correctly placed in the documentation. </review>

Proposal

Discussed in #538977 (comment 2485869445).

We should display the text within <review></review> tag if it is available and if there were no <comment> tags found in the response. The text will be added to the summary comment.

Before:

I finished my review and found nothing to comment on. Nice work! 🎉

After:

I finished my review and found nothing to comment on. Nice work! 🎉

The changes look good. The MR adds the environment variable `export GITLAB_SIMULATE_SAAS=0` to the documentation for setting up a cloud license via CustomersDot. This aligns with the MR description which mentions that this variable is needed when running a self-managed instance to add subscriptions. The change is straightforward and correctly placed in the documentation.

Out of scope

We will not yet specifically instruct the LLM to produce such a summary. This can be a future iteration. Rationale: we are avoiding prompt until the migration to AI GW is complete: #501321 (closed).