Propagate CancellationException in Flow/Suspend helpers; refine timeout handling and dispatcher default

[yangcyzhang]

Motivation

• Ensure CancellationException is never swallowed by Flow helpers so coroutine cancellation and structured concurrency remain correct.
• Avoid treating parent cancellations as timeouts in withTimeoutOrDefault so outer-scope cancellations propagate.
• Make the library compatible with plain JVM usage by defaulting the main dispatcher to Dispatchers.Default and update docs for renamed callback helper.

Description

• Updated Flow extensions to re-throw CancellationException in onEachCatching, flatMapLatestCatching, and mapNotNullCatching while still swallowing other exceptions.
• Changed withTimeoutOrDefault to catch TimeoutCancellationException specifically so parent cancellations are propagated.
• Set CoroutineKitDispatchers.main default to Dispatchers.Default for JVM support.
• Updated README example to use callbackToFlow (renamed helper) and adjusted docs accordingly.
• Added unit tests validating CancellationException propagation for onEachCatching, mapNotNullCatching, flatMapLatestCatching, and parent-cancellation behavior for withTimeoutOrDefault.

Testing

• Ran unit tests with ./gradlew :coroutinekit:test and all tests (including new tests in FlowExtTest and SuspendExtTest) passed.
• Added tests assert CancellationException is propagated for Flow helpers and that withTimeoutOrDefault does not return the fallback when the parent coroutine is cancelled, and they succeeded.

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Codex Task