Add Parallel AI Search grounding tool support

[adek05]

Is your feature request related to a problem? Please describe.
The Vertex AI API supports grounding with Parallel AI Search, but the Go SDK does not expose this functionality. Users who want to use Parallel Web Systems' search API for grounding have to manually construct raw JSON requests instead of using typed Go structs.

Describe the solution you'd like
Add ParallelAiSearch as a new field on the Tool struct, along with supporting config types:

• ParallelAiSearch — top-level tool config with api_key (string) and customConfigs
• ParallelAiSearchCustomConfigs — source_policy, excerpts, max_results
• ParallelAiSearchSourcePolicy — exclude_domains, include_domains (up to 10 each)
• ParallelAiSearchExcerpts — max_chars_per_result, max_chars_total

Transformer behavior: passthrough for Vertex AI, rejection for Gemini API (this is a Vertex AI-only feature).

The response side requires no changes — existing GroundingMetadata types already handle the Parallel AI Search response format.

Describe alternatives you've considered
Manually constructing the parallelAiSearch tool config as map[string]any and passing it through ExtraBody, but this bypasses type safety and is error-prone.

Additional context
Possible implementation in #690

[Sivasankaran25]

@adek05 Thank you for your feedback. We appreciate you taking the time to share your thoughts with us, and we'll be filing a feature request.