fix(sdk-rtl): widen IAccessToken.refresh_token to accept null

[marc-wilson]

What

@looker/sdk and @looker/sdk-rtl both declare an IAccessToken interface with the same name but incompatible shapes for refresh_token:

• @looker/sdk (packages/sdk/src/4.0/models.ts): refresh_token?: string | null — matches the actual Looker API response shape
• @looker/sdk-rtl (packages/sdk-rtl/src/authSession.ts): refresh_token?: string — narrower, re-declared independently of model generation

This meant a login_user() response (typed with the sdk's IAccessToken) couldn't be passed directly into AuthToken.setToken()/constructor (typed with the rtl's IAccessToken) without a manual null -> undefined cast at the call site:

error TS2345: Argument of type '.../@looker/sdk/lib/4.0/models".IAccessToken' is not assignable
to parameter of type '.../@looker/sdk-rtl/lib/authSession".IAccessToken'.
  Types of property 'refresh_token' are incompatible.
    Type 'string | null | undefined' is not assignable to type 'string | undefined'.
      Type 'null' is not assignable to type 'string | undefined'.

Fix

Widen sdk-rtl's IAccessToken.refresh_token to string | null, matching @looker/sdk's shape. @looker/sdk depends on @looker/sdk-rtl (not the reverse), so widening the rtl-side declaration is the only direction that doesn't introduce a circular package dependency.

This is a type-only change with no behavior difference: AuthToken.setToken() already does if (token.refresh_token) { ... }, so null and undefined are already treated identically at runtime — a nil refresh_token means "keep using the existing refresh token."

Verification

• Added packages/sdk/src/4.0/accessTokenCompat.spec.ts, which constructs an @looker/sdk IAccessToken with refresh_token: null and passes it directly into sdk-rtl's AuthToken with no cast.
• Confirmed the fix is load-bearing: temporarily reverted just the one-line type widening and ran npx tsc -p tsconfig.json (full monorepo, source-level typecheck) — reproduced the exact TS2345 error from this issue. Restored the fix and reran — zero errors across the whole repo.
• Full monorepo tsc -p tsconfig.json passes cleanly with the fix in place.
• yarn build passes across all 21 packages.
• Ran full jest suites for sdk, sdk-rtl, sdk-node, hackathon: same pass/fail counts on this branch as on main (548 passed; 5 pre-existing failures require a live looker.ini/Looker connection, unrelated to this change).

Fixes #1729

Test plan

• npx tsc -p tsconfig.json (full monorepo typecheck) passes with zero errors
• yarn build passes cleanly across all 21 packages
• New test in packages/sdk/src/4.0/accessTokenCompat.spec.ts passes
• Confirmed via revert that the fix is what resolves the type error (not incidental)
• No regressions in existing sdk / sdk-rtl / sdk-node / hackathon test suites