fix(omsdk): run OMID activation on the main thread; disable on failure

[JakubMrozek]

Problem

A consumer calls KontextAds.createSession() from a background coroutine (Dispatchers.Default, via ChatViewModel.initializeSdk → KontextAdProvider.initKontext). That ran Omid.activate() off the main thread, where OMID's internal new Handler() throws:

RuntimeException: Can't create handler inside thread … that has not called Looper.prepare()

OMID flips its own isActive flag before that throw, so the SDK was left half-initialized — its activate-guard then no-ops every retry. The next session would start a "valid" OMID session whose process-global TreeWalker dereferences a never-initialized WeakReference<Context>, crashing the app on the main thread:

java.lang.NullPointerException: WeakReference.get() on a null object reference
    at com.iab.omid.library.kontextso.internal.k.a
    at com.iab.omid.library.kontextso.walking.TreeWalker.l

Signature in logs: om-session-started {valid=false} (activation threw) → {valid=true} (next session) → crash.

Fix

OmManager:

• Activation runs on the main thread. When activate() is called off-main it posts Omid.activate() to the main Looper (fire-and-forget, non-blocking — a blocking post-and-await would deadlock a caller that is itself holding the main thread, e.g. runBlocking on main). OMID session start already happens later on the main thread (after preload + ad-done), so the posted activation always completes first.
• Process-global state + sticky kill-switch. globallyActivated / permanentlyUnavailable @Volatile companion flags. If activation ever fails (throws or leaves OMID inactive), OMID is disabled process-wide so the TreeWalker is never armed and the crash is structurally impossible.

No public API changes. For off-main callers, viewability is restored (not merely suppressed).

Verification

Driven through a multi-Activity character-switch repro that calls createSession() off-main (the field-crash pattern), on an emulator with the real OMID AAR:

• Before: valid=false → valid=true → crash.
• After: valid=true on every session across repeated character switches, no crash, no ANR, OMID genuinely active.
• Also verified the no-AAR path: activation fails → kill-switch → valid=false everywhere, no crash.
• ./gradlew :build spotlessCheck detekt (incl. unit tests) green.

Consumer note

Calling createSession() on the main thread is a valid immediate workaround on the integration side; this change makes it safe from any thread. Server-side OM kill-switch (omit the om block from the bid) remains the no-release tourniquet.

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