Verify and document graceful degradation when no slots match the request URL

[prk-Jr]

Context

When no creative-opportunities.toml slots match the incoming request URL, the following guards prevent any partial auction or injection state:

Guard	Location	Effect
should_run_auction = false	publisher.rs:905	No auction dispatched
ad_slots_script = None	publisher.rs:985	window.__ts_ad_slots not injected
winning_bids = {}	publisher.rs:1457	No bid data produced
has_slots guard	html_processor.rs:305	window.__ts_bids script entirely skipped

No \</body> hold occurs. The page streams unmodified. GAM falls back to its own auction.

Action

• Add explicit tests in publisher.rs verifying zero injection when slot match is empty (currently covered implicitly but not as a named case)
• Confirm behavior when creative_opportunities config section is present but creative-opportunities.toml has zero slots (kill-switch path per spec §8)
• Log a debug message at the no-match gate so operators can confirm the feature is inactive on non-article URLs

Parent: #677