Integration + provider layer type migration

[aram356]

Description

Migrate integration layer (IntegrationProxy::handle) and auction provider layer (AuctionProvider::request_bids → async, parse_response takes PlatformResponse) to platform-agnostic types. Migrate send/send_async calls deferred from PR 6.

Scope reduction from PR 12.5

RuntimeServices plumbing is already done in #515 (PR 12.5). This PR focuses on the two remaining concerns:

1. Type migration: fastly::Request/Response → http types across all 8 production + 2 test IntegrationProxy implementations, plus 3 AuctionProvider implementations
2. send/send_async migration: prebid.rs, aps.rs, adserver_mock.rs switch from Fastly SDK send()/send_async() to services.http_client

Sub-slice acceptance checklist

1. Trait signatures — IntegrationProxy and AuctionProvider use http types; request_bids is async fn; parse_response takes PlatformResponse; all impls compile
2. send/send_async migration — no direct fastly::Request::send / send_async remains in any integration module
3. Routing types — IntegrationRegistration and route_request() use http::Method
4. NextJS verification — confirm submodules need zero changes; only mod.rs updated
5. Test parity — no fastly::* test fixtures remain in integration test modules

Done when

• All 5 sub-slice checks pass
• request_bids is async
• send/send_async migrated to services.http_client
• Per-PR gates pass

Blocked by

PR 12.5 (#515)

References

• docs/internal/EDGEZERO_MIGRATION.md — Phase 2, PR 13
• Part of EdgeZero migration #480