chore: Introduction of u-pr-agent and fix of Updated Dependencies Test

[michalChrobot]

Purpose of this PR

This PR follows the introduction of u-pr-agent to the repo

1. I installed the app as described in https://github.cds.internal.unity3d.com/unity/u-pr-agent
2. I added a modification via .pr-agent.toml file so the bot won't run automatically but needs to be triggered via (for example) /improve (see more commands in linked repo)

Additionally I noticed Updated Dependencies Test failures which seem to need project-pack job to work properly (the job was removed in #3711).
This is probably due to some signing that the project pack job were doing. For now I will reintroduce it in this one place but later I will investigate why it happens

Jira ticket

N/A

Documentation

N/A

Testing & QA (How your changes can be verified during release Playtest)

Green pass on Updated Dependencies Test and correct bot trigger via comment

Backports

Backport to develop branch in #3763

[michalChrobot]

/improve

[u-pr-agent]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪ The PR only adds a very simple 2-line configuration file.

🏅 Score: 100 The PR correctly adds a simple configuration file to set up a new tool as described.

🧪 No relevant tests

🔒 No security concerns identified

⚡ No major issues detected

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