Expose session attribution (gclid, UTM, landing page) on text conversations via the API

The v3 API exposes full session attribution on calls and form submissions — gclid, fbclid, utm_source/medium/campaign/content/term, landing_page_url, referrer_domain, and now a milestones object. This works great, and it's how we report called-in and form leads as offline conversions in Google Ads.

Text conversations are the one gap. On /text-messages.json, the fields parameter rejects every attribution field — the allowed set is limited to ids, tracking/customer numbers, state, recent_messages, lead_status, and source. There's also no relational key (pageview_id / visitor_id) to join a text back to its web session on /pageviews.json.

The data clearly exists — CallRail already displays it. Your own Text Message Attribution report breaks texts down by source, campaign, referring page, and landing page. And on the API, the source field on a text conversation is session-derived: we texted a tracking number swapped into a Google-organic web session and the conversation came back with source: "Google Organic". So the session join is already resolved server-side — this is about exposing existing data on one endpoint, not building a new join.

Why it matters: SMS is a primary inbound channel for us, but a texted-in lead's click can never be reported as a Google Ads offline conversion, because Google requires the gclid and the API won't return it for texts. A called-in lead can be closed out; an identical texted-in lead can't — purely because the field isn't exposed. That silently under-reports paid-search performance and starves Google's bidding of real conversion signal.

The ask (any one solves it):

1. Allow the existing attribution fields (gclid, utm_*, landing_page_url, referrer_domain) in fields on /text-messages.json; or
2. Add the same milestones object you just shipped on form submissions to text conversations; or
3. Return a relational session/visitor id on texts so we can fetch the record from /pageviews.json ourselves.

source was added to text conversations in early 2025, and the Text Message Attribution report has shown full source/campaign/landing-page data even longer — the association is already there.