feat(providers): add Perplexity as a declarative provider

[jliounis]

Summary

• Add Perplexity as a declarative OpenAI-compatible provider.
• Include the Sonar models from the maintainer-provided template with PERPLEXITY_API_KEY, streaming support, model docs, and setup steps.
• Add Perplexity to the supported providers documentation.

Context

This follows up on #8920, which was closed because the requested direction was to use goose's JSON-based declarative provider system instead of adding a Rust provider module. The provider implementation delta here is JSON-only, so there is no Rust module or provider-specific Rust test surface to maintain.

Validation

• jq empty crates/goose/src/providers/declarative/perplexity.json
• git diff --check
• Attempted cargo test -p goose declarative_providers; this Buildkite image could not complete it because Hermit expects /bin/ln, the AGI Cargo wrapper points at an internal registry, and an isolated crates.io Cargo run later failed on missing c++ for llama-cpp-sys-2.

Signed-off-by: James Liounis james.liounis@perplexity.ai

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