Delete SSC cookie and KV store entry when user withdraws consent

[ChristianPavilonis]

User story

As a publisher, I want existing SSC cookies and KV store entries to be deleted when a user revokes consent, so that tracking stops immediately upon withdrawal.

Acceptance criteria

• If consent is not given AND an SSC cookie is present → delete the cookie value AND delete the corresponding KV store entry
• Revocation is acted on in the same request where lack of consent is detected
• After revocation, subsequent requests with no consent and no cookie result in no action (do nothing)

Affected area

Core (synthetic IDs, cookies, GDPR), Fastly runtime

Proposed approach

1. After decoding consent (per the TCF/GPP decoding issue), check for the revocation scenario: has_consent == false && ssc_cookie.is_some().
2. If triggered:
   • Set the SSC cookie to expire (clear it from the response).
   • Delete the corresponding entry from the Fastly KV store.
3. If has_consent == false && ssc_cookie.is_none(), do nothing — no cleanup needed.
4. This check runs on every request as part of the consent evaluation pipeline.

Additional context

Sub-issue of #54. Related to #312, PR #380.
Handles the consent withdrawal / revocation case required by GDPR Article 7(3) and US state privacy laws.