feat(server): default request validation to 'warn' in dev/test

[bokelley]

Summary

• Flip createAdcpServer's request validation default from 'off' to 'warn' when NODE_ENV !== 'production'. Production default stays 'off' (zero AJV overhead).
• Mirrors the asymmetric response-side default shipped in Tighten generated types + default response validation to catch schema drift at the right layer #727 A (strict/off). Warn mode logs a single Schema validation warning (request) line per mismatched payload via the configured logger and still dispatches — nothing is rejected.
• Completes the original intent of feat(validation): schema-driven validation with client hooks and opt-in server middleware #694 (which called for requests: 'warn' everywhere in its body but shipped 'off' as code/docs drift).

Why

Symmetric defaults with responses, and prepares seller operators for upstream schema tightenings — notably adcp#2795, which introduces a required asset_type discriminator on creative assets. Buyer agents still on RC3 fixtures will lack it; surfacing that as a warning during handler development beats discovering it in a downstream VALIDATION_ERROR after deploy.

Opt-out

• Per-server: validation: { requests: 'off' } on the createAdcpServer config.
• Global: set NODE_ENV=production for the process.

Test plan

• npm run build:lib passes
• npm run test:lib passes (4391 tests, 0 failures, 6 skipped)
• schema-validation-server.test.js — the renamed requests: "off" test now asserts explicit opt-out instead of relying on it being the default
• prettier --check clean on modified files
• Manual verification: construct a server without validation: under NODE_ENV=test, send a request that fails the schema, confirm a Schema validation warning (request) line appears in the logger and the handler still runs
• Manual verification: set validation: { requests: 'off' } and confirm no warning fires
• CI green

Refs: #694 (original intent), #727 A (response-side precedent), adcp#2795 (upstream asset_type discriminator this prepares for).