Document bulk companies API (Preview)

[VarshSuresh]

Why?

The bulk companies API is now available in Preview but has no OpenAPI coverage yet, so the endpoints aren't discoverable in the Preview reference docs.

How?

Documents four operations in the Preview spec, following the existing bulk contacts pattern:

• POST /companies/bulk — bulk create or update, matched on the company_id you define (parity with the single create-or-update company endpoint).
• PUT /companies/bulk — bulk update by company id; unknown ids fail per-item and never create.
• POST /companies/bulk/delete — bulk delete by company id; detaches associated contacts while keeping the company record.
• GET /companies/bulk/{id} — bulk job status.

Adds the three bulk request schemas and a companies_bulk_job response schema. Each request is an async job (max 100 items); a failed item ends the job as completed_with_errors without failing the batch.

Companion to:

• https://github.com/intercom/developer-docs/pull/1064

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