Pass user agent to auction telemetry

[ChristianPavilonis]

What

Passes the request User-Agent through each auction observation and every emitted Tinybird row.

• /auction reads the value from AuctionRequest.device.user_agent.
• Initial-navigation and SPA auction paths snapshot the original request header.
• auction_events_raw gains a nullable user_agent string column.
• Missing or non-string header values remain null.

This deliberately does not classify browser_family at the edge. Downstream data syncs own browser aggregation, as requested in the review of #1039.

Contract change

The complete User-Agent is now intentionally retained in raw auction telemetry, including skipped and consent-denied observations, under the datasource's existing 30-day TTL. The telemetry design and implementation plan now document this contract. EC IDs, internal auction request IDs, IP addresses, full URLs, query strings, and fragments remain excluded.

Why

Edge-side browser classification fixes the aggregation policy in application code and discards information that downstream data syncs may need. Keeping the source value lets each sync derive the browser dimensions it requires without another edge deployment.

Supersedes #1039 and addresses its requested change.

Related to #979

Verification

• Added failing-before regression coverage for complete User-Agent NDJSON serialization.
• cargo fmt --all -- --check
• cargo test-fastly
• cargo test-axum
• cargo test-cloudflare
• cargo test-spin
• cargo test --manifest-path crates/trusted-server-integration-tests/Cargo.toml --test parity
• cargo clippy-fastly
• cargo clippy-axum
• cargo clippy-cloudflare
• cargo clippy-cloudflare-wasm
• cargo clippy-spin-native
• cargo clippy-spin-wasm
• cd crates/trusted-server-js/lib && npx vitest run
• cd crates/trusted-server-js/lib && npm run format
• cd docs && npm run format
• tb check tinybird/datasources/auction_events_raw.datasource

Rollout

Deploy the additive Tinybird datasource column before deploying the edge version. Older rows and requests without a valid User-Agent remain null.