feat(provider): Add Perplexity Agent API provider

[kesku]

Issue for this PR

Closes #15921

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a custom loader for the perplexity-agent provider so it uses sdk.responses() instead of the default sdk.languageModel() fallback. Also adds the standard HTTP-Referer / X-Title headers consistent with other providers.

Depends on anomalyco/models.dev#1082 which defines the provider metadata, model list, base URL, and PERPLEXITY_API_KEY env var.

How did you verify your code works?

Ran the models.dev API locally from anomalyco/models.dev#1082, set OPENCODE_MODELS_URL to point at the local instance, and confirmed the provider and its models appear correctly and requests route to the right endpoint.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Add Perplexity Agent API 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 updating your PR! It now meets our contributing guidelines. 👍