Stop logging static creative-rewriting warnings on every request

[ChristianPavilonis]

Description

When auctions are enabled with rewrite_creatives = false, settings loading emits:

WARN [settings] Auction creative rewriting disabled; creative assets and clicks may contact third-party hosts directly

The warning currently lives in crates/trusted-server-core/src/settings.rs after settings validation. On Fastly, that path runs during ordinary request handling, so a static configuration choice floods runtime logs. A 50-second production audit captured 198 copies of this message. It accounted for most of the 262 warnings in the window and obscured actionable EID, Prebid Server, and Sourcepoint failures.

rewrite_creatives = false may be an intentional operator choice. Repeating the same warning does not provide request-level information.

Expected behavior

Remove this warning from runtime settings loading. Either eliminate it or emit it once from the CLI when validating or pushing a configuration that has both:

[auction]
enabled = true
rewrite_creatives = false

If retained in the CLI, the message should explain the third-party asset and click behavior without blocking publication. One CLI invocation should produce at most one warning.

This change must not alter creative rewriting defaults or runtime behavior.

Completion criteria

• Fastly request handling no longer emits the static warning.
• If the warning moves to the CLI, ts config validate or ts config push reports it once for the applicable configuration.
• Configurations with auctions disabled or creative rewriting enabled do not report it.
• Core and CLI tests cover the selected behavior.