fix(oauth): advertise offline_access without letting it touch the tier clamp (#159)

[ifahimreza]

Closes #159

What

Both OAuth discovery documents now list offline_access in scopes_supported, and a client that sends that scope, at authorize or at refresh, is treated exactly as if it had not: it is never mapped to a tier, and it never changes what is granted.

Why

OpenAI's "Developer mode and MCP apps in ChatGPT" page (re-read 2026-09-07) says ChatGPT inspects a provider's discovery metadata for offline_access before trusting refresh-token renewal, and that without it "ChatGPT may lose access after the original authorization expires". Saddle issues refresh tokens unconditionally and advertises the refresh_token grant, but never listed the scope. With ACCESS_TTL = 3600 the failure mode is a connector that works for one hour and then needs reconnecting, which is indistinguishable from the "connected but broken" reports already being fielded.

How

• Saddle_OAuth::REFRESH_SCOPE and advertised_scopes(). SCOPES is untouched and a test pins it at exactly three; the tier clamp never sees the new word.
• Saddle_OAuth::strip_refresh_scope() runs before the authorize endpoint's "did the client name a scope?" decision. Without it, a client sending only offline_access would have been treated as one that asked for openid profile: pinned to read forever, the pre-fix(mcp): answer the handshake the way strict clients require (#97) #98 ChatGPT outcome all over again.
• The same strip on the refresh path. Without it a refresh naming only offline_access normalized to saddle:read and silently narrowed an admin grant on renewal.
• Nothing is echoed back: the granted scope never contains offline_access, so the invariant that every granted scope is a subset of SCOPES holds.

Testing

• composer lint — 0 errors
• composer test — 657 tests (was 654), 1 pre-existing skip
• Four new tests: discovery lists it in both documents; offline_access alone reaches consent proposing the site's own level; saddle:read offline_access is not widened; a refresh naming only offline_access keeps the granted scope
• Empirical: connect a ChatGPT Business workspace and confirm the connection survives past one hour — needs an account we do not have on hand