feat: integrate FingerprintJS Pro visitor header for bot detection

[thostetler]

• Added fingerprint_core.js utility (mirrors recaptcha_core.js pattern): lazy-loads the FingerprintJS Pro CDN script, initializes the SDK, and caches the visitorId
• Wired into discovery_bootstrap.configure() where DynamicConfig is available and api.modifyRequestOptions is set; header injection lives inside the existing override closure with a first-party URL guard
• Added fingerprintApiKey config var to discovery.vars.js.default (empty = disabled, no script loaded)
• All failure paths (script load, SDK init, fp.get()) resolve to null and are swallowed — this cannot break requests or halt bootstrap under any circumstances

[Copilot]

Pull request overview

This PR adds a best-effort FingerprintJS Pro integration to Discovery so the client can attach a per-visitor identifier header to first-party ADS API requests (to support downstream bot-detection/traffic analysis) without impacting bootstrap or request execution if anything fails.

Changes:

• Added a new fingerprint_core utility that lazy-loads the FingerprintJS Pro CDN script, initializes the SDK, and caches visitorId.
• Wired the utility into discovery_bootstrap.configure() to (a) start loading the SDK when fingerprintApiKey is configured and (b) inject X-Fp-Visitor-Id on first-party API requests.
• Added fingerprintApiKey to the default Discovery vars and added Mocha tests for the new utility.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/js/utils/fingerprint_core.js	New utility to inject FingerprintJS Pro script and cache visitorId.
src/js/mixins/discovery_bootstrap.js	Loads FingerprintJS Pro at bootstrap and adds visitor header to first-party API requests.
src/config/discovery.vars.js.default	Adds fingerprintApiKey config knob (empty disables).
test/mocha/js/utils/fingerprint_core.spec.js	Adds unit tests covering load success/failure paths and caching behavior.

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