Add linkPolicy to HighlightsContentsOptions

[LiamHz]

Summary

Adds linkPolicy option to highlights configuration across all SDKs and documentation. This allows users to control whether links are stripped or kept in highlight snippets.

Changes:

• monorepo: Added linkPolicy?: "strip" | "keep" to HighlightsContentsOptions in shared types
• docs: Added linkPolicy enum field to highlights schema in exa-spec.yaml
• exa-js: Added linkPolicy to HighlightsContentsOptions type with JSDoc
• exa-py: Added link_policy: Literal["strip", "keep"] to HighlightsContentsOptions

The backend (vulcan) already supports this field via UrlPolicySchema in requests.ts.

Review & Testing Checklist for Human

• Verify backend actually processes linkPolicy correctly (vulcan has the schema but confirm it's wired through to snippet generation)
• Test with linkPolicy: "strip" via API call to confirm links are removed from highlights
• Confirm default behavior matches docs ("keep" by default)

Notes

This is a type-only change exposing an existing backend capability to SDK users.

Link to Devin run: https://app.devin.ai/sessions/3d9a8bd68822476a9383ea8a4631e8e1
Requested by: liam@exa.ai (@LiamHz)

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