fix(tabs): restore findTabById undefined contract

[atldays]

Problem

findTabById(tabId) currently returns getTab(tabId) without await inside its try block. Because getTab() rejects asynchronously, the catch is never reached.

Observed consequences:

• a missing tab rejects instead of returning undefined as documented;
• the dedicated Tab id "..." not exist branch in getTabUrl() is unreachable;
• callers observe the lower-level browser error instead.

Proposed follow-up

Change the implementation to return await getTab(tabId) and add focused regression coverage for both findTabById() and getTabUrl().

This is intentionally separate from the framework-agnostic testing-kit work because it changes observable behavior of two public functions.