Build a standalone validation tool for configuring creative opportunities #701

Context

Current validation of creative-opportunities.toml is minimal:

• Build-time only: slot ID charset check (^[A-Za-z0-9_-]+$) in build.rs
• validate_slot_id() exists in creative_opportunities.rs but is not called at runtime
• No CI check, no standalone tool, no auditor-facing feedback

This creates friction for ad ops and JS auditors (raised by Aram) who need to validate slot config changes without running a full Rust build.

Missing validations

• Duplicate slot IDs
• Pattern overlap / unreachable patterns (e.g. /* shadows /article/**)
• Format size sanity (width > 0, height > 0)
• floor_price >= 0
• Bidder names against the [integrations.prebid].bidders list
• APS slot_id presence when APS is enabled
• PBS stored-request cross-check: does each slot ID have a corresponding entry in the configured PBS instance?

Proposed tool

A ts-validate CLI (or cargo xtask validate) that:

1. Parses creative-opportunities.toml and runs all structural checks
2. Optionally probes PBS for stored-request existence (requires PBS URL in config)
3. Outputs a human-readable report suitable for ad ops review
4. Exits non-zero on any error — runnable in CI without a full WASM build

Action

• Define the full validation rule set
• Implement as a standalone binary or xtask
• Add to CI pipeline alongside cargo clippy and cargo test

Parent: #677