fix: rotate explicit Build proxy pools per request

[AmPlace]

What changed

• mark explicit Build proxy-pool leases as requiring a fresh connection tunnel
• disable connection reuse only for those Build requests
• preserve keep-alive behavior for fixed proxies, Web/Console scopes, and account-bound proxy templates
• add regression tests for the affected and unaffected paths

Root cause

Rotating proxy endpoints commonly choose an exit IP when a new CONNECT tunnel is established. The shared Build HTTP transport reused keep-alive/HTTP2 connections, so multiple requests could remain pinned to one exit even when the egress node was explicitly configured as a proxy pool.

Impact

Explicit Build proxy pools establish a fresh tunnel per request, matching their configured rotation semantics. The additional handshake cost is limited to this mode.

Checks

• go test ./internal/application/account ./internal/application/gateway ./internal/infra/egress