feat: add PKCE OAuth boundary
Introduce provider-neutral HTTP contracts and public-client authorization-code orchestration with PKCE S256, strict HTTPS and loopback validation, constant-time state checks, secure OpenSSL entropy, and single-send token exchange semantics.
Add independent protocol and adversarial suites covering RFC 7636, RFC3986 encoding, callback failures, response redaction, and no-retry behavior. Concrete networking, browser/listener, token parsing, Secret Service, and Graph integration remain separate follow-up slices.
Bernardo Magri
dd6e02cb55