[Cobalt] Add URL sanitization for URLs provided by the platform.

[jshorta]

Use deep link resolution for URLs that don't start with the YTLR prefix.

Bug: 503024637

[github-actions]

🤖 Gemini Suggested Commit Message

---

cobalt: Sanitize platform provided startup URLs

In release builds, enforce that the startup URL starts with the
predefined default URL. If a non-compliant URL is provided by the
platform, treat it as a deep link and reset the startup URL to the
default. This ensures that the application always boots into a
consistent, verified state while still supporting deep links.

Bug: 503024637

---

💡 Pro Tips for a Better Commit Message:

1. Influence the Result: Want to change the output? You can write custom prompts or instructions directly in the Pull Request description. The model uses that text to generate the message.
2. Re-run the Generator: Post a comment with: /generate-commit-message

[gemini-code-assist]

Code Review

This pull request enforces that the startup URL points strictly to YouTube TV in release builds, rerouting invalid URLs to a deep link and sanitizing the arguments. The review feedback correctly identifies a critical lifetime issue where 'initial_deep_link' could become a dangling pointer by referencing a temporary local string, and suggests using a reference to 'init_argv.back()' instead. It also recommends adding a defensive check to ensure 'args' is not empty before accessing 'args.back()'.