fix: validate cookie methods and preserve setAll receiver

[VG31OP]

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

1. When cookie accessor methods (setAll, get, set, remove) are explicitly set to undefined (or non-function values) on the cookies options object, the library previously only used the in operator ("setAll" in cookies, "set" in cookies, etc.). Because "key" in { key: undefined } evaluates to true in JavaScript:
2. • In createServerClient, omitting setAll logs a helpful warning on token refresh, but passing { getAll: () => [], setAll: undefined } (common with conditional adapter configurations) bypassed this fallback and threw TypeError: originalSetAll is not a function.
3. • In createBrowserClient, passing { getAll: () => [], setAll: undefined } bypassed the initialization check and crashed later on storage write with TypeError: setAll is not a function.
4. • Passing cookies: { get: () => ..., set: undefined, remove: undefined } attempted to call cookies.set and threw TypeError: cookies.set is not a function.
5. 2. When custom stateful cookie adapters rely on this, calling setAll as an unbound reference drops its method receiver, throwing TypeError: Cannot set properties of undefined (fixing cookies.setAll loses its receiver for stateful cookie adapters #299).
6. 3. In createServerClient, the internal setAll wrapper calls Object.keys(headers).length. If headers is omitted or nullish, it crashes with TypeError: Cannot convert undefined or null to object.

What is the new behavior?

• Validates that cookie methods (get, getAll, set, remove, setAll) are actual functions before attempting invocation, preserving in for TypeScript union type narrowing while guaranteeing runtime safety when properties are explicitly undefined.
• • Preserves the cookie adapter receiver when invoking cookies.setAll.
• • Guards the setAll server wrapper with Boolean(headers && Object.keys(headers).length > 0) to handle omitted or optional headers cleanly.
• • Preserves the existing graceful server fallback warning when setAll is undefined, and ensures createBrowserClient enforces setAll presence at construction time.

Verification

• pnpm exec vitest run: 125/125 tests pass (5 new unit tests added covering all cases).
• • pnpm run build: both tsc builds (ESM & CJS) compile cleanly with 0 errors.
• • pnpm exec eslint: 0 lint errors or warnings.
• • pnpm exec prettier --check src/cookies.ts src/cookies.spec.ts src/createBrowserClient.spec.ts: cleanly formatted.