fix: scope the OBS token cache to each LINE login

[Fizzadar]

The OBS access token cache was a package-level global, so every LINE login in a process sent whichever login's OBS token was cached last as x-line-access, and any login's re-authentication cleared it for all of them. This moves the cache onto LineClient and attaches it to every line.Client that login creates, so caching and invalidation are per login.

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• Indent Review Agent · Every OBS path now uses its login's cache, and I found no regressions or nil-cache risks.

Full results

Indent Review Agent

• Summary: Every OBS path now uses its login's cache, and I found no regressions or nil-cache risks.
• Last ran on commit: 3b9e5c50
• Latest result

  {
    "summary": "Every OBS path now uses its login's cache, and I found no regressions or nil-cache risks.",
    "findings": []
  }