feat: add LLM Gateway provider

[smakosh]

What does this PR do?

• Adds LLM Gateway as a built-in provider

How did you verify your code works?

• Tested it locally: https://x.com/smakosh/status/2010429712590651640

Fixes: #7848

[github-actions]

Hey! Your PR title Add LLM Gateway provider doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search results, I found one potentially related PR:

Related PR:

• Add support for LLM Gateway w/ Anthropic #933: Add support for LLM Gateway w/ Anthropic
  • This appears to be an older PR that adds LLM Gateway support, though it specifically mentions Anthropic integration. It may overlap with or be a predecessor to the current feature being added in PR feat: add LLM Gateway provider #7847.

The other result (#4477 - Gatewayz.ai) is a different gateway provider and not directly related.